Handle missing Lua when lua backend is enabled

The first case handles ./configure --with-modules='lua' --with-lua=no.
I don't know who would do such a thing, but better handle it in
./configure than bailing out later with a compiler error.

The second case handles ./configure --with-modules=lua when you don't have
Lua installed, which I actually observed since I assumed enabling the
lua backend would be enough, and I did't have the lua headers on my
system.

Fixes #1618
Merge pull request #1788 from rubenk/handle-missing-lua-better

Handle missing Lua when lua backend is enabled