missing bits/syscall.h when cross-building scanf15.c test

[Nathan Lynch]

With current master (6f12d0629f41) I get the following error when trying
to run "make check":

$ printf "CC=arm-linux-gnueabihf-gcc\nBUILD_CC=gcc\n" > configparms
$ CC=arm-linux-gnueabihf-gcc ../../../src/glibc.git/configure \
  --prefix=/usr --disable-profile --host=arm-linux-gnueabihf \
  --with-headers=/srv/dev/build/linux/arm/headers/include
...
$ make
$ make test-wrapper='/srv/dev/src/glibc.git/scripts/cross-test-ssh.sh
imx6sl' check
...
arm-linux-gnueabihf-gcc scanf15.c -c -std=gnu99 -fgnu89-inline  -O2
-Wall -Winline -Wundef -Wwrite-strings -fmerge-all-constants
-frounding-math -g -Wstrict-prototypes   -I../libio -I../stdlib
-I../wcsmbs -I../time -I../string -I../wctype       -I../include
-I/srv/dev/build/glibc/armhf/stdio-common  -I/srv/dev/build/glibc/armhf
 -I../sysdeps/unix/sysv/linux/arm  -I../nptl/sysdeps/unix/sysv/linux
-I../sysdeps/unix/sysv/linux  -I../sysdeps/nptl  -I../sysdeps/pthread
-I../sysdeps/gnu  -I../sysdeps/unix/inet  -I../nptl/sysdeps/unix/sysv
-I../sysdeps/unix/sysv  -I../sysdeps/unix/arm  -I../nptl/sysdeps/unix
-I../sysdeps/unix  -I../sysdeps/posix  -I../sysdeps/arm/armv7/multiarch
 -I../sysdeps/arm/armv7  -I../sysdeps/arm/armv6t2
-I../sysdeps/arm/armv6  -I../sysdeps/arm/nptl  -I../sysdeps/arm/include
-I../sysdeps/arm  -I../sysdeps/arm/soft-fp  -I../sysdeps/wordsize-32
-I../sysdeps/ieee754/flt-32  -I../sysdeps/ieee754/dbl-64
-I../sysdeps/ieee754  -I../sysdeps/generic  -I../nptl  -I.. -I../libio
-I. -nostdinc -isystem
/srv/dev/toolchain/gcc-linaro-arm-linux-gnueabihf-4.8-2013.11_linux/bin/../lib/gcc/arm-linux-gnueabihf/4.8.3/include
-isystem
/srv/dev/toolchain/gcc-linaro-arm-linux-gnueabihf-4.8-2013.11_linux/bin/../lib/gcc/arm-linux-gnueabihf/4.8.3/include-fixed
-isystem /srv/dev/build/linux/arm/headers/include  -D_LIBC_REENTRANT
-include ../include/libc-symbols.h   -DNOT_IN_libc=1    -D_IO_MTSAFE_IO
-o /srv/dev/build/glibc/armhf/stdio-common/scanf15.o -MD -MP -MF
/srv/dev/build/glibc/armhf/stdio-common/scanf15.o.dt -MT
/srv/dev/build/glibc/armhf/stdio-common/scanf15.o
In file included from ../misc/sys/param.h:28:0,
                 from ../include/sys/param.h:1,
                 from ../sysdeps/unix/sysv/linux/arm/lowlevellock.h:22,
                 from ../sysdeps/nptl/bits/stdio-lock.h:23,
                 from ../libio/libio.h:149,
                 from ../libio/stdio.h:74,
                 from scanf15.c:10:
../include/signal.h:44:15: warning: ‘struct sigvec’ declared inside
parameter list [enabled by default]
        struct sigvec *__ovec);
               ^
../include/signal.h:44:15: warning: its scope is only this definition or
declaration, which is probably not what you want [enabled by default]
In file included from ../sysdeps/unix/sysdep.h:20:0,
                 from ../sysdeps/unix/arm/sysdep.h:18,
                 from ../sysdeps/unix/sysv/linux/arm/sysdep.h:24,
                 from ../sysdeps/unix/sysv/linux/arm/lowlevellock.h:25,
                 from ../sysdeps/nptl/bits/stdio-lock.h:23,
                 from ../libio/libio.h:149,
                 from ../libio/stdio.h:74,
                 from scanf15.c:10:
../sysdeps/unix/sysv/linux/sys/syscall.h:31:27: fatal error:
bits/syscall.h: No such file or directory
 # include <bits/syscall.h>
                           ^
compilation terminated.
make[2]: *** [/srv/dev/build/glibc/armhf/stdio-common/scanf15.o] Error 1


Is this user error?

Removing the "#undef _LIBC" from scanf15.c works around it, fwiw.
scanf17 is similarly affected.