x11-fonts/fontconfig: Use triggers to rebuild fc-cache

Stop forcing the overwrite of the cache at each run it is not necessary
most of the time.

Reviewed by: tcberner
Differencial Revision: https://reviews.freebsd.org/D34602

While here rework USES=fonts to drop @fcfontsdir which is not nessary anymore
and convert the @fontsdir into lua which make it way simpler to read