Allow NVCCFLAGS to customize compiler invocations similar to CFLAGS, CXXFLAGS etc.
Allow NVCCFLAGS to customize compiler invocations similar to CFLAGS, CXXFLAGS etc.

f0k/cudamat,zhmz90/cudamat,cudamat/cudamat,surban/cudamat,RaoUmer/cudamat,NervanaSystems/cudamat