Ifdefs, moved semantic to check, fixed error reporting for builtins

test/ifdefs.ink

@@ -1,5 +1,17 @@
-#if Defines.Skinning {
-	vertex main :: () {
-		// Stub
+//#add_define Skinning
+#add_define UV
+
+
+#if Env.Skinning {
+	vertex skinning_main :: () {
+		x : float = 5.0;
+	}
+} else #if Env.UV {
+	vertex texture_mapping_main :: () {
+		x : float2 = float2(2.0, 2.0);
 	}
 }
+
+pixel main :: () {
+
+}