Add a flag for elective compilation without CUDA.

The existing CMake scripts were building InfiniTAM with CUDA if CUDA_FOUND
was set to true, and without CUDA otherwise. This generally works well,
but makes it difficult to build without CUDA by choice. This commit adds a
WITH_CUDA flag that is by default set to the value of CUDA_FOUND, but can
be overridden by the person configuring InfiniTAM. The commit also
modifies the various CMake scripts to use this flag rather than CUDA_FOUND.

contributed by stuart golodetz