Allow Terra to initialize when LLVM is not build with NVPTX backend

[elliottslaughter]

Right now we have a hard dependency on the NVPTX backend of LLVM even when CUDA is not enabled in Terra. This may potentially limit what LLVM binaries we can compile against. If we make the initialization conditional on CUDA support being enabled then we could build with more pre-built LLVM binaries.

Failure looks like:

terra/src/terralib.lua:747: failed to initialize target for LLVM Triple: nvptx64-nvidia-cuda (No available targets are compatible with this triple.)

[astrolemonade]

Can we make this dependency optional before version 1 is released? A lot of people will consider using it and will have this problem when trying terra.

[elliottslaughter]

As an update, we only initialize the target when terralib.cudahome is set, but that is currently unconditional on Linux. We should really make it smarter, because it's not as if we'll use this target if Terra isn't even built with CUDA.

I've added this to #492.

[elliottslaughter]

This was fixed in #541.