Libchula's test complains with HAVE_STRCASESTR

When cmake finds strcasestr and defines HAVE_STRECASETR then the compiler complains about `implicit declaration of function ‘strcasestr’` in *libchula/test/missing_sysfuncs_test.c*.

The reason for this is that the function declaration is conditioned to *__USE_GNU* in *string.h*.

All that's needed to fix this is defining the macro _GNU_SOURCE before any #includes.