package/ffmpeg: add optional dependency to libxcb

ffmpeg 3.3 added optional support for libxcb:
https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/HEAD:/Changelog#l282

This patch ensures a reproducible build of libavdevice.so:

$ readelf -a output/target/usr/lib/libavdevice.so | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libavfilter.so.7]
 0x0000000000000001 (NEEDED)             Shared library: [libavformat.so.58]
 0x0000000000000001 (NEEDED)             Shared library: [libavcodec.so.58]
 0x0000000000000001 (NEEDED)             Shared library: [libavutil.so.56]
 0x0000000000000001 (NEEDED)             Shared library: [libxcb.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libxcb-shm.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libxcb-shape.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libxcb-xfixes.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.0]

compared to

$ readelf -a output/target/usr/lib/libavdevice.so | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libavfilter.so.7]
 0x0000000000000001 (NEEDED)             Shared library: [libavformat.so.58]
 0x0000000000000001 (NEEDED)             Shared library: [libavcodec.so.58]
 0x0000000000000001 (NEEDED)             Shared library: [libavutil.so.56]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.0]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/ffmpeg/Config.in

@@ -67,6 +67,13 @@ config BR2_PACKAGE_FFMPEG_FFPROBE
 	  FFprobe is a utility to determine the audio and video
 	  characteristics of a container file.
 
+config BR2_PACKAGE_FFMPEG_XCBGRAB
+	bool "Build X11 screen grabbing"
+	depends on BR2_PACKAGE_XORG7
+	select BR2_PACKAGE_LIBXCB
+	help
+	  X11 screen grabbing using libxcb.
+
 config BR2_PACKAGE_FFMPEG_AVRESAMPLE
 	bool "Build libavresample"
 	help

package/ffmpeg/ffmpeg.mk

@@ -96,6 +96,17 @@ else
 FFMPEG_CONF_OPTS += --disable-ffprobe
 endif
 
+ifeq ($(BR2_PACKAGE_FFMPEG_XCBGRAB),y)
+FFMPEG_CONF_OPTS += \
+	--enable-libxcb \
+	--enable-libxcb-shape \
+	--enable-libxcb-shm \
+	--enable-libxcb-xfixes
+FFMPEG_DEPENDENCIES += libxcb
+else
+FFMPEG_CONF_OPTS += --disable-libxcb
+endif
+
 ifeq ($(BR2_PACKAGE_FFMPEG_POSTPROC),y)
 FFMPEG_CONF_OPTS += --enable-postproc
 else