meson: Always define _GNU_SOURCE for pthread checks

Without this, GNU-specific symbols won't be defined and the compiler
check will pass because GCC will assume that you know what you're
doing since it doesn't know what the symbol prototype is and compiler
checks aren't built with -Wall -Werror.

This will then cause a build failure because the wrong prototype will
be used.

meson.build

@@ -1272,12 +1272,17 @@ else
   if cc.has_header_symbol('pthread.h', 'pthread_cond_timedwait_relative_np')
     glib_conf.set('HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE_NP', 1)
   endif
-  if cc.links('''#include <pthread.h>
+  # Assume that pthread_setname_np is available in some form; same as configure
+  if cc.links('''#ifndef _GNU_SOURCE
+              # define _GNU_SOURCE
+              #endif
+              #include <pthread.h>
               int main() {
                 pthread_setname_np("example");
               }''',
               name : 'pthread_setname_np(const char*)',
               dependencies : thread_dep)
+    # macOS and iOS
     glib_conf.set('HAVE_PTHREAD_SETNAME_NP_WITHOUT_TID', 1)
   elif cc.links('''#ifndef _GNU_SOURCE
                 # define _GNU_SOURCE
@@ -1288,6 +1293,7 @@ else
                 }''',
                 name : 'pthread_setname_np(pthread_t, const char*)',
                 dependencies : thread_dep)
+    # Linux, Solaris, etc.
     glib_conf.set('HAVE_PTHREAD_SETNAME_NP_WITH_TID', 1)
   endif
 endif