Add option helper for MESON_ARGS = -Dfoo=enabled

feature options define combo like auto/yes/no in a standand way.
In other words, upstream projects of ports that use _MESON_YES are
supposed to migrate to _MESON_ENABLED.

https://mesonbuild.com/Build-options.html#features

Reviewed by: tobik (implicit)
Approved by: portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D19127