Add support for USES=meson and option ${opt}_MESON_* handling.
I added a new port graphics/graphene for testing and demonstration,
and adapted multimedia/gstreamer1 as another example. Both ports have
a few issues but I believe those are port related andnot meson itself.
Also both ports will be committed at another time.
Open problems:
- meson complaining about using US-ASCII locale, but it really wants a Unicode compatible. I assume that commands are run in a "clean" enviroment. Because it doesn't pick up my LANG=en_US.UTF-8, not sure how to handle it.
- Currently the --strip command is hidden behind WITH_DEBUG, is this ok?
- graphene: introspection support is always on, ignore the comments.
- graphene: GTKDOC option has both _ON and _TRUE for demonstration.