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>

shared/meson.build

@@ -10,7 +10,7 @@ deps_libshared = dep_wayland_client
 lib_libshared = static_library(
 	'shared',
 	srcs_libshared,
-	include_directories: [ include_directories('..'), public_inc ],
+	include_directories: common_inc,
 	dependencies: deps_libshared,
 	pic: true,
 	install: false
@@ -68,7 +68,7 @@ endif
 lib_cairo_shared = static_library(
 	'cairo-shared',
 	srcs_cairo_shared,
-	include_directories: include_directories('..'),
+	include_directories: common_inc,
 	dependencies: deps_cairo_shared,
 	install: false
 )