Use gcc-specific pragma only on gcc (and the family)

spec/ruby/optional/capi/ext/thread_spec.c

@@ -19,7 +19,9 @@ static VALUE thread_spec_rb_thread_alone(VALUE self) {
   return rb_thread_alone() ? Qtrue : Qfalse;
 }
 
+#if defined(__GNUC__)
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+#endif
 
 /* This is unblocked by unblock_func(). */
 static void* blocking_gvl_func(void* data) {