build: add Meson fallback for wayland-protocols

This allows easily co-developing a Wayland protocol and Weston.
Example setup:

    ln -s subprojects/wayland-protocols /path/to/wayland-protocols
    meson configure build/ --force-fallback-for=wayland-protocols

Signed-off-by: Simon Ser <contact@emersion.fr>

meson.build

@@ -6,7 +6,7 @@ project('weston',
 		'c_std=gnu99',
 		'b_lundef=true',
 	],
-	meson_version: '>= 0.52.1',
+	meson_version: '>= 0.54.0',
 	license: 'MIT/Expat',
 )
 

protocol/meson.build

@@ -1,8 +1,9 @@
 dep_scanner = dependency('wayland-scanner', native: true)
 prog_scanner = find_program(dep_scanner.get_pkgconfig_variable('wayland_scanner'))
 
-dep_wp = dependency('wayland-protocols', version: '>= 1.24')
-dir_wp_base = dep_wp.get_pkgconfig_variable('pkgdatadir')
+dep_wp = dependency('wayland-protocols', version: '>= 1.24',
+	fallback: ['wayland-protocols', 'wayland_protocols'])
+dir_wp_base = dep_wp.get_variable(pkgconfig: 'pkgdatadir', internal: 'pkgdatadir')
 
 install_data(
 	[