[Bug string/25399] New: undefined reference to `__warn_memset_zero_len' when changing gnuc version

["sguelton at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=25399

            Bug ID: 25399
           Summary: undefined reference to `__warn_memset_zero_len' when
                    changing gnuc version
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: string
          Assignee: unassigned at sourceware dot org
          Reporter: sguelton at redhat dot com
  Target Milestone: ---

When compiling the following program

```
#undef __GNUC__
#define __GNUC__ 4
#undef __GNUC_MINOR__
#define __GNUC_MINOR__ 9

#include <string.h>
char a[64];
int main ()
{
  memset (a, 64, 0);
  return 0;
}
```

with a decent gcc version (8.3) and glibc version (2.28), I get a link error

```
gcc a.c -D_FORTIFY_SOURCE=2 -O2
In file included from /usr/include/string.h:494,
                 from a.c:6:
In function ‘memset’,
    inlined from ‘main’ at a.c:10:3:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:67:7: warning: call to
‘__warn_memset_zero_len’ declared with attribute warning: memset used with
constant zero length parameter; this could be due to transposed parameters
       __warn_memset_zero_len ();
       ^~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/ld: /tmp/cc7RDLx1.o: in function `main':
a.c:(.text.startup+0x5): undefined reference to `__warn_memset_zero_len'
collect2: error: ld returned 1 exit status
```

No link error if I don't force the __GNUC__ and __GNUC_MINOR__.


I've checked the source of glibc-2.30, and the problem seems related to

```
#if !__GNUC_PREREQ (5,0)
__warndecl (__warn_memset_zero_len,
      "memset used with constant zero length parameter; this could be due to
transposed parameters");
#endif
```

while `__warn_memset_zero_len` is not defined elsewhere.

In addition to binding glibc to some gcc version, it is also a problem for
non-gcc compilers, like clang, which set GNU C version to 3.2 by default (see
for instance https://reviews.llvm.org/D71082#1822925)

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