Makefile improvements

[forklady42]

Addresses two pieces of the Makefile.

First, a fix for #51. Instead of hardcoding CUDA_PATH and related variables before attempting to detect the CUDA_PATH, moved the default CUDA_PATH into the CUDA_PATH detection conditional. Then, set the renaming variables. This way differing CUDA paths in other environments aren't overwritten.

Second, updated make rebuild to call setup-full rather than setup, as setup was previously removed.