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/kernel.c

@@ -4,6 +4,7 @@
 #include <assert.h>
 #include <math.h>
 
+#include "compiler.h"
 #include "kernel.h"
 #include "utils.h"