meson: Convert v4l2 into a feature option

The v4l2 compatibility layer does not have external dependencies,
the usual benefits of a feature option do not apply. The main
motivation behind this change is making use of the `auto_features`
meson option that can change all feature options set to "auto"
by default to the desired value.

This can be useful for two reasons: (1) using auto_features=disabled
and then building up the list of features that are desired in the
particular build; (2) using auto_features=enabled to achieve
(usually) more testing and compilation coverage.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

meson_options.txt

@@ -84,6 +84,7 @@ option('udev',
        description : 'Enable udev support for hotplug')
 
 option('v4l2',
-        type : 'boolean',
-        value : false,
-        description : 'Compile the V4L2 compatibility layer')
+        type : 'feature',
+        value : 'auto',
+        description : 'Compile the V4L2 compatibility layer',
+        deprecated : {'true': 'enabled', 'false': 'disabled'})

src/v4l2/meson.build

@@ -1,12 +1,11 @@
 # SPDX-License-Identifier: CC0-1.0
 
-if not get_option('v4l2')
-    v4l2_enabled = false
+v4l2_enabled = get_option('v4l2').allowed()
+
+if not v4l2_enabled
     subdir_done()
 endif
 
-v4l2_enabled = true
-
 v4l2_compat_sources = files([
     'v4l2_camera.cpp',
     'v4l2_camera_file.cpp',