GST-ffmpeg comes bundled with an internal ffmpeg that suffers from
the known issue of a hardcoded gcc unless the right configure arg
is thrown. When building with clang, the hardcoded gcc in the
libav subdirectory causes an error where gcc doesn't recognize
`-march=native`. GST-ffmpeg offers a configure flag to fix that.
Add a flag to use our ENV.cc.
FixesHomebrew/homebrew#12312ClosesHomebrew/homebrew#12390.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>
