wscript: refactor opus wscript files, now it builds only if system package wasn't found

wscript

@@ -54,6 +54,7 @@ class Subproject:
 		return True
 
 SUBDIRS = [
+	Subproject('3rdparty/opus'),
 	Subproject('public',      dedicated=False, mandatory = True),
 	Subproject('filesystem',  dedicated=False, mandatory = True),
 	Subproject('game_launch', singlebin=True),
@@ -353,11 +354,9 @@ int main(int argc, char **argv) { strcasestr(argv[1], argv[2]); return 0; }'''
 			continue
 
 		conf.add_subproject(i.name)
-	
-	conf.load('opus')
 
 def build(bld):
-	bld.load('opus xshlib')
+	bld.load('xshlib')
 
 	for i in SUBDIRS:
 		if not i.is_enabled(bld):