Fix possible segmentation fault in strnstr() on 64-bit systems

As of f22054c (Modify strnstr.c., 2017-08-30), the strnstr()
implementation was replaced by a version that segfaults (at least
sometimes) on 64-bit systems.

Without the _GNU_SOURCE constant defined, the prototype of memmem() is
missing, and as a consequence its return type defaults to int (and GCC
spits out a warning).

On 64-bit systems, the int data type is too small, though, to hold a
full char *, hence the upper 32-bit are cut off and bad things happen
due to a bogus pointer being used to access memory.

Reported as https://github.com/Alexpux/MINGW-packages/issues/2879 in
the MSYS2 project.

Cc: Sichen Zhao <1473996754@qq.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

newlib/libc/string/strnstr.c

@@ -31,6 +31,7 @@ QUICKREF
 */
 
 #undef __STRICT_ANSI__
+#define _GNU_SOURCE
 #include <_ansi.h>
 #include <string.h>