Add a workaround for incorrect CUDA_HOST_COMPILER setting

Recent versions of cmake set CUDA_HOST_COMPILER to CMAKE_C_COMPILER
which on OSX defaults to clang (/usr/bin/cc), but this is not a
supported cuda compiler.  This commit adds a workaround to
preemptively set CUDA_HOST_COMPILER to gcc if that compiler exists in
/usr/bin.
Merge pull request #32 from patmarion/0979-fix-cuda-host-compiler

Add a workaround for incorrect CUDA_HOST_COMPILER setting