Single Compilation Unit build.

Adds support for simple SCU build.
This speeds up compilation by compiling multiple cpp files within a single translation unit.

main/tests/test_string.cpp

@@ -1330,3 +1330,5 @@ MainLoop *test() {
 	return nullptr;
 }
 } // namespace TestString
+
+#undef CHECK

main/tests/test_theme.cpp

@@ -301,3 +301,5 @@ MainLoop *test() {
 	return nullptr;
 }
 } // namespace TestTheme
+
+#undef CHECK