ffmpeg: fix pkg-config utilization

in config.log we can see:
WARNING: aarch64-linaro-linux-pkg-config not found, library detection may fail.

ffmpeg configure script is not looking for pkg-config at the rigt place since it
is assuming cross compilation. let's force its value in the recipe.

This patches 'fixes' library detection, so it also adds:

--disable-libxcb
--disable-libxcb-shm
--disable-libxcb-xfixes
--disable-libxcb-shape

Which were dangling configure options, which started to be enabled after the
pkg-config fix, so they need now to be explicitely disabled. Follow up patch
will enable these options when DISTRO_FEATURES has x11.

(From OE-Core rev: 3d5f11f0a1fd036e28a1d3f0c3169d8e21cc1358)

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-multimedia/ffmpeg/ffmpeg_3.3.2.bb

@@ -88,6 +88,10 @@ EXTRA_OECONF = " \
     --enable-pic \
     --enable-shared \
     --enable-pthreads \
+    --disable-libxcb \
+    --disable-libxcb-shm \
+    --disable-libxcb-xfixes \
+    --disable-libxcb-shape \
     ${@bb.utils.contains('USE_NONFREE', 'yes', '--enable-nonfree', '', d)} \
     \
     --cross-prefix=${TARGET_PREFIX} \
@@ -108,6 +112,7 @@ EXTRA_OECONF = " \
     --datadir=${datadir}/ffmpeg \
     ${@bb.utils.contains('AVAILTUNES', 'mips32r2', '', '--disable-mipsdsp --disable-mipsdspr2', d)} \
     --cpu=${@cpu(d)} \
+    --pkg-config=pkg-config \
 "
 
 EXTRA_OECONF_append_linux-gnux32 = " --disable-asm"