meson.build: improve handling of vapi
As it currently stands the vapi files are not generatable on linux distros which do not provide unversioned valac/vapigen. Use instead meson's add_languages function to handle detection of existing vala toolchain, which will check the VALAC and VAPIGEN env vars.
Signed-off-by: Marty E. Plummer hanetzer@startmail.com
Edited by hanetzer