For Python, Lua and Javascript, make the option boolean (with 'yes'
being the default). No need of a warning when not installing the
plug-ins as this would have been disabled explicitly anyway. When
installing the plug-ins, only make interpreter checks as precautionnary
verifications which don't actually change anything (except outputting
some warnings if interpreters are not found). Basically for these 3
bindings, the interpreters are only runtime dependencies anyway. So it
doesn't matter if they are not available at build time. In particular,
we get rid of the 'force' option.

Vala rules do not change as the vala compiler is indeed needed at build
time and current checks work correctly. I just add a "Vala plug-ins"
line in the summary message of the meson configuration, as it was
missing.