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/config.h

@@ -7,9 +7,10 @@
 /// Common functions and definitions for configuration parsing
 /// Used for command line arguments and config files
 
+#include <stdlib.h>
 #include <stdbool.h>
 #include <ctype.h>
-#include <strings.h>
+#include <string.h>
 #include <xcb/xfixes.h>
 
 #ifdef CONFIG_LIBCONFIG