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>

fullscreen-shell/meson.build

@@ -7,7 +7,7 @@ if get_option('shell-fullscreen')
 	shared_library(
 		'fullscreen-shell',
 		srcs_shell_fullscreen,
-		include_directories: include_directories('..', '../shared'),
+		include_directories: common_inc,
 		dependencies: dep_libweston,
 		name_prefix: '',
 		install: true,