meson: refactor generating config.h

mpv has a ton of defines that are generated during building. Previously,
the meson build just had this as a big giant wall of text that manually
set each one but we can do this smarter. Instead, change the "features"
object to a dictionary and have it hold the name of the feature and its
value (true/false on whether it is enabled). Then at the end, just loop
through it and reformat the name of the feature so it becomes
HAVE_FEATURE. A side effect of this is that a lot of extra defines are
generated that aren't actually used in the code, but the waf build
worked like this for years anyway. A nice result of this is that the use
of foo['use'] internally can be completely eliminated and replaced with
feature['foo'] instead when needed.