ffmpeg: enable use of freetype and fontconfig

Configure ffmpeg to use freetype and fontconfig
if those packages are selected.

ffmpeg freetype support require fenv.h, which is not available in uClibc or
musl, except for certain archs / depending on specific configuration
options, so only enable it for glibc.

[Peter: only enable on glibc, simplify logic]
Signed-off-by Stephen M. Kenton <skenton@ou.edu>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>