Meson options are typed, so the prefix isn't necessary, and indeed
looks odd when used to disable the option:
-Denable_foo=false
Also replace underscores with dashes, which is the preferred meson
style.
https://bugzilla.gnome.org/show_bug.cgi?id=791007
Meson is on track to replace autotools as the build system of choice,
so support it in addition to autotools. If all goes well, we'll
eventually be able to drop the latter ...
https://bugzilla.gnome.org/show_bug.cgi?id=783229
