I get a build failure in the sunrpc/ subdirectory when building GLIBC on Alpine Linux 3.6.1.

The toolchain has a target of `x86_64-alpine-linux-musl` so it's perhaps not officially supported by GLIBC. However this issue is the only thing that prevents GLIBC 2.25 from compiling in that environment so it would be worthwhile to fix it.

musl libc defines certain types in its sys/types.h when _GNU_SOURCE or _BSD_SOURCE is set, such as `u_char`, `u_short`, and `caddr_t`.

See: http://git.musl-libc.org/cgit/musl/tree/include/sys/types.h#n59

These are redefined inside GLIBC in `sunrpc/rpc/types.h`, causing 'conflicting types' issues.

If GLIBC could detect the existance of these symbols at configure-time in the host libc it could then #ifdef away the redefinitions appropriately.

This is the full error output:

    gcc   -D_RPC_THREAD_SAFE_ -D_GNU_SOURCE -DIS_IN_build -include /buildstream/build/o/config.h rpc_clntout.c \
        -o /buildstream/build/o/sunrpc/cross-rpc_clntout.o -MMD -MP -MF /buildstream/build/o/sunrpc/cross-rpc_clntout.o.dt -MT /buildstream/build/o/sunrpc/cross-rpc_clntout.o -c
    In file included from rpc_parse.c:39:0:
    rpc/types.h:78:9: error: unknown type name '__u_char'
     typedef __u_char u_char;
             ^~~~~~~~
    rpc/types.h:78:18: error: conflicting types for 'u_char'
     typedef __u_char u_char;
                      ^~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_parse.c:39:
    /usr/include/sys/types.h:64:23: note: previous declaration of 'u_char' was here
     typedef unsigned char u_char;
                           ^~~~~~
    In file included from rpc_parse.c:39:0:
    rpc/types.h:79:9: error: unknown type name '__u_short'
     typedef __u_short u_short;
             ^~~~~~~~~
    rpc/types.h:79:19: error: conflicting types for 'u_short'
     typedef __u_short u_short;
                       ^~~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_parse.c:39:
    /usr/include/sys/types.h:65:24: note: previous declaration of 'u_short' was here
     typedef unsigned short u_short, ushort;
                            ^~~~~~~
    In file included from rpc_parse.c:39:0:
    rpc/types.h:80:9: error: unknown type name '__u_int'
     typedef __u_int u_int;
             ^~~~~~~
    rpc/types.h:80:17: error: conflicting types for 'u_int'
     typedef __u_int u_int;
                     ^~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_parse.c:39:
    /usr/include/sys/types.h:66:18: note: previous declaration of 'u_int' was here
     typedef unsigned u_int, uint;
                      ^~~~~
    In file included from rpc_parse.c:39:0:
    rpc/types.h:81:9: error: unknown type name '__u_long'
     typedef __u_long u_long;
             ^~~~~~~~
    rpc/types.h:81:18: error: conflicting types for 'u_long'
     typedef __u_long u_long;
                      ^~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_parse.c:39:
    /usr/include/sys/types.h:67:23: note: previous declaration of 'u_long' was here
     typedef unsigned long u_long, ulong;
                           ^~~~~~
    In file included from rpc_parse.c:39:0:
    rpc/types.h:82:9: error: unknown type name '__quad_t'
     typedef __quad_t quad_t;
             ^~~~~~~~
    rpc/types.h:82:18: error: conflicting types for 'quad_t'
     typedef __quad_t quad_t;
                      ^~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_parse.c:39:
    /usr/include/sys/types.h:68:19: note: previous declaration of 'quad_t' was here
     typedef long long quad_t;
                       ^~~~~~
    In file included from rpc_parse.c:39:0:
    rpc/types.h:83:9: error: unknown type name '__u_quad_t'
     typedef __u_quad_t u_quad_t;
             ^~~~~~~~~~
    rpc/types.h:83:20: error: conflicting types for 'u_quad_t'
     typedef __u_quad_t u_quad_t;
                        ^~~~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_parse.c:39:
    /usr/include/sys/types.h:69:28: note: previous declaration of 'u_quad_t' was here
     typedef unsigned long long u_quad_t;
                                ^~~~~~~~
    In file included from rpc_parse.c:39:0:
    rpc/types.h:84:9: error: unknown type name '__fsid_t'
     typedef __fsid_t fsid_t;
             ^~~~~~~~
    rpc/types.h:88:9: error: unknown type name '__daddr_t'
     typedef __daddr_t daddr_t;
             ^~~~~~~~~
    rpc/types.h:90:9: error: unknown type name '__caddr_t'
     typedef __caddr_t caddr_t;
             ^~~~~~~~~
    rpc/types.h:90:19: error: conflicting types for 'caddr_t'
     typedef __caddr_t caddr_t;
                       ^~~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_parse.c:39:
    /usr/include/sys/types.h:63:15: note: previous declaration of 'caddr_t' was here
     typedef char *caddr_t;
                   ^~~~~~~
    In file included from rpc_clntout.c:34:0:
    rpc/types.h:78:9: error: unknown type name '__u_char'
     typedef __u_char u_char;
             ^~~~~~~~
    rpc/types.h:78:18: error: conflicting types for 'u_char'
     typedef __u_char u_char;
                      ^~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_clntout.c:34:
    /usr/include/sys/types.h:64:23: note: previous declaration of 'u_char' was here
     typedef unsigned char u_char;
                           ^~~~~~
    In file included from rpc_clntout.c:34:0:
    rpc/types.h:79:9: error: unknown type name '__u_short'
     typedef __u_short u_short;
             ^~~~~~~~~
    rpc/types.h:79:19: error: conflicting types for 'u_short'
     typedef __u_short u_short;
                       ^~~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_clntout.c:34:
    /usr/include/sys/types.h:65:24: note: previous declaration of 'u_short' was here
     typedef unsigned short u_short, ushort;
                            ^~~~~~~
    In file included from rpc_clntout.c:34:0:
    rpc/types.h:80:9: error: unknown type name '__u_int'
     typedef __u_int u_int;
             ^~~~~~~
    rpc/types.h:80:17: error: conflicting types for 'u_int'
     typedef __u_int u_int;
                     ^~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_clntout.c:34:
    /usr/include/sys/types.h:66:18: note: previous declaration of 'u_int' was here
     typedef unsigned u_int, uint;
                      ^~~~~
    In file included from rpc_clntout.c:34:0:
    rpc/types.h:81:9: error: unknown type name '__u_long'
     typedef __u_long u_long;
             ^~~~~~~~
    rpc/types.h:81:18: error: conflicting types for 'u_long'
     typedef __u_long u_long;
                      ^~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_clntout.c:34:
    /usr/include/sys/types.h:67:23: note: previous declaration of 'u_long' was here
     typedef unsigned long u_long, ulong;
                           ^~~~~~
    In file included from rpc_clntout.c:34:0:
    rpc/types.h:82:9: error: unknown type name '__quad_t'
     typedef __quad_t quad_t;
             ^~~~~~~~
    rpc/types.h:82:18: error: conflicting types for 'quad_t'
     typedef __quad_t quad_t;
                      ^~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_clntout.c:34:
    /usr/include/sys/types.h:68:19: note: previous declaration of 'quad_t' was here
     typedef long long quad_t;
                       ^~~~~~
    In file included from rpc_clntout.c:34:0:
    rpc/types.h:83:9: error: unknown type name '__u_quad_t'
     typedef __u_quad_t u_quad_t;
             ^~~~~~~~~~
    rpc/types.h:83:20: error: conflicting types for 'u_quad_t'
     typedef __u_quad_t u_quad_t;
                        ^~~~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_clntout.c:34:
    /usr/include/sys/types.h:69:28: note: previous declaration of 'u_quad_t' was here
     typedef unsigned long long u_quad_t;
                                ^~~~~~~~
    In file included from rpc_clntout.c:34:0:
    rpc/types.h:84:9: error: unknown type name '__fsid_t'
     typedef __fsid_t fsid_t;
             ^~~~~~~~
    rpc/types.h:88:9: error: unknown type name '__daddr_t'
     typedef __daddr_t daddr_t;
             ^~~~~~~~~
    rpc/types.h:90:9: error: unknown type name '__caddr_t'
     typedef __caddr_t caddr_t;
             ^~~~~~~~~
    rpc/types.h:90:19: error: conflicting types for 'caddr_t'
     typedef __caddr_t caddr_t;
                       ^~~~~~~
    In file included from rpc/types.h:69:0,
                     from rpc_clntout.c:34:
    /usr/include/sys/types.h:63:15: note: previous declaration of 'caddr_t' was here
     typedef char *caddr_t;
                   ^~~~~~~
    make[2]: *** [Makefile:167: /buildstream/build/o/sunrpc/cross-rpc_parse.o] Error 1
    make[2]: *** Waiting for unfinished jobs....
    make[2]: *** [Makefile:167: /buildstream/build/o/sunrpc/cross-rpc_clntout.o] Error 1
    make[2]: Leaving directory '/buildstream/build/sunrpc'
    make[1]: *** [Makefile:215: sunrpc/others] Error 2
    make[1]: Leaving directory '/buildstream/build'
    make: *** [Makefile:9: all] Error 2