Draft: meson: avoid misleading subproject error
Whenever we set required to true here, we'll get an error about a missing wrap file from Meson, which isn't terribly clear.
But we verify that LLVM is found further down in the build-script in all
of these cases apart from the _llvm == 'enabled' case, so we can just
remove them.
If a user specified -Dllvm=true, and didn't provide anything else about
where to find LLVM, this confusing error might still occur. But it's not
too far off; this is the right place in the sources to start digging,
and adding the wrap-file might be exactly what they want.
Edited by Erik Faye-Lund