package/dvdauthor: add optional support for libfribidi

When libfribidi was compiled before, dvdauthor will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/spumux | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libdl.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libexpat.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libfontconfig.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libfreetype.so.6]
 0x0000000000000001 (NEEDED)             Shared library: [libfribidi.so.0]
[...]

The build system offers no option to en-/disable libfribidi support:
https://github.com/ldo/dvdauthor/blob/master/configure.ac#L83



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>