meson: fix broken boolean kwarg

meson: fix broken boolean kwarg

Everywhere else that `conf.get('ENABLE_*')` is used as a boolean key for
something (for example in if statements) it always checks if == 1, but
in this one case it neglects to do so. This is important because
conf.get yields the same int that was stored, but if statements require
booleans.

So does executable's "install" kwarg, at least according to the
documentation. In actuality, it accepts all types without sanity
checking, then uses python "if bool(var)", so you can actually do
`install: 'do not'` and that's treated identical to `true`. This is a
type-checking bug which Meson will eventually fix.

muon fails on the same code, today.