Enable Werror for gcc

Resolves #15 (closed)

I've enabled Werror for gcc.

I tried -Werror all-warnings for NVCC following the docs: https://docs.nvidia.com/cuda/cuda-compiler-driver-nvcc/index.html
But it seems that's for a later version of NVCC. We can revisit after the upgrade. There's a few other 'phases' of compilation that have similar flags, and we might as well enable those too, but at the moment we don't have any warnings to test those on. I'd use #78 (closed) to track this issue.

I've tested by adding the LIGOLwReader.c warning back in, and it errored as expected.