meson: fix libtool versioning

libtool versions must be translated to maj.min.rev, where maj = current - age.

Also fix lt_current usage, quoting Samuel Thibault:

soversion is not the same as lt_current. soversion must be lt_current-lt_age, so that the soname stays the same when binary compatibility is preserved (here, 0), and gets bumped on compatibility break. Yes, on compatibility break the bump will be big. That's because non-soname OSes the compatibility support is different. The details are hairy, yes :)

Instead, rely on meson doing the right thing with soversion by default.