Fix 'MADV_DONTNEED undeclared' compiler error in os_dep.c on Cosmo

(fix of commit a72ac8b)

Issue #666 (bdwgc).

Note: the error was observed when compiling on Darwin only.

* include/private/gc_priv.h [!__linux__ && __COSMOPOLITAN__
&& USE_MUNMAP && !_GNU_SOURCE] (_GNU_SOURCE): Define.

include/private/gc_priv.h

@@ -28,6 +28,7 @@
 
 #if (defined(__linux__) || defined(__GLIBC__) || defined(__GNU__) \
      || defined(__CYGWIN__) || defined(HAVE_DLADDR)               \
+     || (defined(__COSMOPOLITAN__) && defined(USE_MUNMAP))        \
      || defined(GC_HAVE_PTHREAD_SIGMASK)                          \
      || defined(HAVE_PTHREAD_SETNAME_NP_WITHOUT_TID)              \
      || defined(HAVE_PTHREAD_SETNAME_NP_WITH_TID_AND_ARG)         \