Add description for meson options
The descriptions are displayed in the result of:
mesonconf builddir
-option('build_app', type: 'boolean', value: true)
-option('build_server', type: 'boolean', value: true)
-option('prebuilt_server', type: 'string')
-option('override_server_jar', type: 'string')
-option('skip_frames', type: 'boolean', value: true)
+option('build_app', type: 'boolean', value: true, description: 'Build the client')
+option('build_server', type: 'boolean', value: true, description: 'Build the server')
+option('prebuilt_server', type: 'string', description: 'Path of the prebuilt server')
+option('override_server_jar', type: 'string', description: 'Hardcoded path to find the server at runtime')
+option('skip_frames', type: 'boolean', value: true, description: 'Always display the most recent frame')
