* Force the recipe for Lua to use the spack compiler.
I'm not sure how the old recipe worked for anyone. The Lua Makefiles set
`CC=gcc` and for my spack environment the first `gcc` found in my `PATH` is
`$SPACK_ROOT/lib/spack/env/gcc`, which is a directory. This caused the build
to fail. My change drops the `-std=gnu99`, but this option doesn't appear
to be required for a sucessful build.
* Preserve the '-std=gnu99' compile option.
05c6c40e36