testsuite: Define _POSIX_C_SOURCE for test

As the tests assume that strndup() is visible (only part of
POSIX.1-2008) define the guard to ensure that it's visible.  Currently,
glibc appears to always have this defined in C++, newlib does not.

Without this patch, fails like this can be seen:

Testing analyzer/strndup-1.c,  -std=c++98
.../strndup-1.c: In function 'void test_1(const char*)':
.../strndup-1.c:11:13: error: 'strndup' was not declared in this scope; did you mean 'strncmp'?
.../strndup-1.c: In function 'void test_2(const char*)':
.../strndup-1.c:16:13: error: 'strndup' was not declared in this scope; did you mean 'strncmp'?
.../strndup-1.c: In function 'void test_3(const char*)':
.../strndup-1.c:21:13: error: 'strndup' was not declared in this scope; did you mean 'strncmp'?

Patch has been verified on Linux.

gcc/testsuite/ChangeLog:

	* c-c++-common/analyzer/strndup-1.c: Define _POSIX_C_SOURCE.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>

gcc/testsuite/c-c++-common/analyzer/strndup-1.c

 /* { dg-skip-if "no strndup in libc" { *-*-darwin[789]* *-*-darwin10* hppa*-*-hpux* *-*-mingw* *-*-vxworks* } } */
+/* { dg-additional-options "-D_POSIX_C_SOURCE=200809L" } */
 
 #include <string.h>
 #include <stdlib.h>