Add compilation flag for V4L2 feature

This allows to disable V4L2 support on Linux to build without
libavdevice.

app/meson.build

@@ -69,7 +69,7 @@ else
     endif
 endif
 
-v4l2_support = host_machine.system() == 'linux'
+v4l2_support = get_option('v4l2') and host_machine.system() == 'linux'
 if v4l2_support
     src += [ 'src/v4l2_sink.c' ]
 endif

app/src/cli.c

@@ -1549,7 +1549,8 @@ parse_args_with_getopt(struct scrcpy_cli_args *args, int argc, char *argv[],
                 opts->v4l2_device = optarg;
                 break;
 #else
-                LOGE("V4L2 (--v4l2-sink) is only available on Linux.");
+                LOGE("V4L2 (--v4l2-sink) is disabled (or unsupported on this "
+                     "platform).");
                 return false;
 #endif
             case OPT_V4L2_BUFFER: