Improve the header includes, cont'd

Also check in the modulemap file, and add a option to build with clang's
-fmodules.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/backend.h

@@ -4,6 +4,7 @@
 #pragma once
 
 #include "region.h"
+#include "compiler.h"
 
 typedef struct session session_t;
 typedef struct win win;