Remove a FIXME and unused variables in Meson

Two meson patches.

One of them adds version gates to two LLVM flags (-frwapv,
-fno-strict-aliasing). I believe we moved the minimum LLVM version
recently, so these might not be necessary, but maybe it helps for
historictal reasons. If not, I'll just remove the comment in a different
patch.

Second patch removes some unused variables. Were they analogous to
things in autotools and the Meson portions haven't been added yet?

I was looking into adding LLVM JIT support to Meson since there is
a TODO about it, but it wasn't clear what was missing except adding some
variables into the PGXS Makefile.

--
Tristan Partin
Neon (https://neon.tech)