Fix 'implicit declaration of pthread_sigmask' compiler warning

Fix 'implicit declaration of pthread_sigmask' compiler warning
(fix commit 1554297)

* include/gc_pthread_redirects.h [GC_NO_PTHREAD_SIGMASK]: Include
signal.h even GC_pthread_sigmask is not declared (otherwise
pthread_sigmask is redefined to GC_pthread_sigmask first and then
signal.h is included thus causing REAL(pthread_sigmask) to stay
undefined).
* include/gc_pthread_redirects.h [GC_NO_PTHREAD_SIGMASK]
(GC_pthread_sigmask): Define it also if _BSD_SOURCE or _GNU_SOURCE.
* tests/test.c [CPPCHECK] (GC_PTHREAD_SIGMASK_NEEDED): Do not define
(because defining _GNU_SOURCE is sufficient to have GC_pthread_sigmask
declared).
* tests/test.c [CPPCHECK] (_GNU_SOURCE): Do not define unless GC_PTHREADS.