libass: new package

libass is a portable subtitle renderer for the
ASS/SSA (Advanced Substation Alpha/Substation Alpha) subtitle format.
This package was originally found at : https://github.com/huceke/buildroot-rbp


By gimli <ebsi4711@gmail.com>

[Peter: make fontconfig/enca optional, fix license, add host-pkgconf,
 optionaliconv/harfbuzz support]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: gimli <ebsi4711@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>