Rule out gcc on Windows

It seems the combination fails at the moment.  Don't know exactly
why but I suspect there can be issues in resolving weak references.
Let's rule them out for now.

https://ci.appveyor.com/project/ruby/ruby/builds/27036383/job/x3c5d54839aacoyt

include/ruby/intern.h

@@ -1171,4 +1171,19 @@ __attribute__((__unused__,__weakref__("rb_define_singleton_method"),__nonnull__(
 #endif
 #endif
 
+#ifdef _WIN32
+#ifdef rb_define_method_id
+#undef rb_define_method_id
+#endif
+#ifdef rb_define_private_function
+#undef rb_define_private_function
+#endif
+#ifdef rb_define_protected_function
+#undef rb_define_protected_function
+#endif
+#ifdef rb_define_singleton_function
+#undef rb_define_singleton_function
+#endif
+#endif
+
 #endif /* RUBY_INTERN_H */

include/ruby/ruby.h

@@ -2818,4 +2818,16 @@ __attribute__((__unused__,__weakref__("rb_define_global_function"),__nonnull__(1
 #endif
 #endif
 
+#ifdef _WIN32
+#ifdef rb_define_method
+#undef rb_define_method
+#endif
+#ifdef rb_define_module_function
+#undef rb_define_module_function
+#endif
+#ifdef rb_define_global_function
+#undef rb_define_global_function
+#endif
+#endif
+
 #endif /* RUBY_RUBY_H */