Autodetect libxcb.

configure

@@ -251,7 +251,7 @@ External library support:
   --enable-libx264         enable H.264 encoding via x264 [no]
   --enable-libx265         enable HEVC encoding via x265 [no]
   --enable-libxavs         enable AVS encoding via xavs [no]
-  --enable-libxcb          enable X11 grabbing using XCB [no]
+  --enable-libxcb          enable X11 grabbing using XCB [auto]
   --enable-libxcb-shm      enable X11 grabbing shm communication [auto]
   --enable-libxcb-xfixes   enable X11 grabbing mouse rendering [auto]
   --enable-libxvid         enable Xvid encoding via xvidcore,
@@ -5024,11 +5024,12 @@ fi
 enabled xlib &&
     check_lib X11/Xlib.h XOpenDisplay -lX11 || disable xlib
 
-if enabled libxcb || enabled x11grab && ! disabled libxcb; then
+if ! disabled libxcb; then
     check_pkg_config xcb xcb/xcb.h xcb_connect || {
         enabled libxcb && die "ERROR: libxcb not found";
     } && disable x11grab && enable libxcb
 
+if enabled libxcb; then
     disabled libxcb_shm ||
         check_pkg_config xcb-shm xcb/shm.h xcb_shm_attach || {
             enabled libxcb_shm && die "ERROR: libxcb_shm not found";
@@ -5042,6 +5043,7 @@ if enabled libxcb || enabled x11grab && ! disabled libxcb; then
     add_cflags "$xcb_event_cflags $xcb_shm_cflags $xcb_xfixes_cflags"
     add_extralibs "$xcb_event_libs $xcb_shm_libs $xcb_xfixes_libs"
 fi
+fi
 
 if enabled x11grab; then
     enabled xlib || die "ERROR: Xlib not found"