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>

remoting/meson.build

@@ -22,7 +22,7 @@ if get_option('remoting')
 	plugin_remoting = shared_library(
 		'remoting-plugin',
 		'remoting-plugin.c',
-		include_directories: include_directories('..', '../shared'),
+		include_directories: common_inc,
 		dependencies: deps_remoting,
 		name_prefix: '',
 		install: true,