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>

meson_options.txt

@@ -13,3 +13,5 @@ option('xrescheck', type: 'boolean', value: false, description: 'Enable X resour
 option('build_docs', type: 'boolean', value: false, description: 'Build documentation and man pages')
 
 option('new_backends', type: 'boolean', value: false, description: 'Does not really do anything right now')
+
+option('modularize', type: 'boolean', value: false, description: 'Build with clang\'s module system')