Fix finalize.c compilation in 'strict ANSI' mode

Fix finalize.c compilation in 'strict ANSI' mode

This and some other GC .c files include gc_pmark.h directly (not
gc_priv.h), so the compiler complains that sigset_t (used in
gc_pthread_redirects.h) is undefined if 'strict ANSI' mode is on and
_GNU_SOURCE is not defined before including system headers.

* include/private/gc_pmark.h [(__linux__ || __GLIBC__ || __GNU__)
&& GC_PTHREADS && !GC_NO_PTHREAD_SIGMASK] (_GNU_SOURCE): Define (before
including other headers).