darwin: be less annoying about "incompatible" library versions

After #3316 we set a default minimum version, now this will warn if you
link with a library that is targeting later versions.

This might be a bit annoying, especially when the user hasn't actually
given Odin a minimum target.

So this PR makes these warnings only show when you explicitly give a
target version (afaik that is the only thing that -mmacosx-min-version
actually does for us because we don't use it to compile anything, just
to link).

src/build_settings.cpp

@@ -436,7 +436,9 @@ struct BuildContext {
 	BlockingMutex target_features_mutex;
 	StringSet target_features_set;
 	String target_features_string;
+
 	String minimum_os_version_string;
+	bool   minimum_os_version_string_given;
 };
 
 gb_global BuildContext build_context = {0};
@@ -1419,7 +1421,7 @@ gb_internal void init_build_context(TargetMetrics *cross_target, Subtarget subta
 
 	bc->metrics = *metrics;
 	if (metrics->os == TargetOs_darwin) {
-		if (bc->minimum_os_version_string.len == 0) {
+		if (!bc->minimum_os_version_string_given) {
 			bc->minimum_os_version_string = str_lit("11.0.0");
 		}