ffmpeg-config: support special 'librtmp_magic' option
Add support for a special option named 'librtmp_magic'. This will not
be passed literally to FFmpeg; instead the script will try to set all
options required to enable librtmp support. The options set are
'--enable-librtmp', '--extra-cflags=' and '--extra-libs='. The last
two will have suitable values determined with pkg-config.
This is meant to make enabling librtmp easier; just '--enable-librtmp'
doesn't work because FFmpeg's configure lacks pkg-config support and
so fails to determine the required flags automatically.