libass: add PACKAGECONFIG for harfbuzz

* it's autodetected from sysroot
* add PACKAGECONFIG to make it deterministic
* runtime dependencies for glib-2.0 harfbuzz libffi libicudata libicule libicuuc libstdc++ are added
  only when harfbuzz is detected

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>