build: simplify include_directories

Define common_inc which includes both public_inc and the project root directory.
The project root directory will allow access to config.h and all the shared/
headers.

Replacing all custom '.', '..', '../..', '../shared' etc. include paths with
common_inc reduces clutter in the target definitions and enforces the common
 #include directive style, as e.g. including shared/ headers without the
subdirectory name no longer works.

Unfortunately this does not prevent one from using private libweston headers
with the usual include pattern for public headers.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston-desktop/meson.build

@@ -15,7 +15,7 @@ srcs_libdesktop = [
 lib_desktop = shared_library(
 	'weston-desktop-@0@'.format(libweston_major),
 	srcs_libdesktop,
-	include_directories: include_directories('..', '../shared'),
+	include_directories: common_inc,
 	install: true,
 	version: '0.0.@0@'.format(libweston_revision),
 	dependencies: dep_libweston