meson: allow selection of components to build

To aid integration, allow meson options to select which components
should be built.

e.g. if a board is not building weston, it's package recipe in it's
build system can disable weston producer, but still get the gpu
producer.
Signed-off-by: Robert Beckett <bob.beckett@collabora.com>

meson_options.txt

+option('weston', type: 'boolean', value: true, description: 'Whether to build weston producer')
+option('gpu', type: 'boolean', value: true, description: 'Whether to build gpu producer')
+option('test', type: 'boolean', value: true, description: 'Whether to build tests')