ffmpeg: Pas CC and CXX to configure

This helps in compiling it with with toolchain coming from
a sstate server where its built using a different build time
sysroot.

Secondly, also helps compiling with non-gcc ( clang ) compiler

(From OE-Core rev: 25deaf1368cc0a99d7b5b3f2d08d7fead51296e2)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-multimedia/ffmpeg/ffmpeg_3.1.1.bb

@@ -76,6 +76,8 @@ EXTRA_OECONF = " \
     --cross-prefix=${TARGET_PREFIX} \
     \
     --ld="${CCLD}" \
+    --cc="${CC}" \
+    --cxx="${CXX}" \
     --arch=${TARGET_ARCH} \
     --target-os="linux" \
     --enable-cross-compile \