Add USES for meson build system (with options)

[kwm]

Summary

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.

Test Plan

Build both graphene and gstreamer1 with meson.
Used GTKDOC _ON en _TRUE lines in graphene to test if they work.

[kwm]

Drop MESON_ENV: not used.
Add USE_LOCALE=en_US.UTF-8 to prevent meson from complaining about a

non-unicode locale. Allow users to overwrite it.

Fix graphene build in poudriere

[swills]

Remove the gstreamer1 bit until that's fixed, otherwise looks fine to me. Tested locally and it works fine, I see now issues with it.

[antoine]

The TRUE_OFF and FALSE_OFF lines look really wrong.

[antoine]

Also the documentation says that --${content} is added when -D${content}= is added

[antoine]

accept so it can be closed

[kwm]

Handled antoine's comments via IRC before I noticed that the review was updated.