Add a testing mode that ensures CPU only code compiles w/o nvcc

[jrhemstad]

As a Thrust user using host-only constructs I expect to be able to compile it without nvcc (i.e., using gcc or clang directly). However, this doesn't always work (see NVIDIA/thrust#386).

We should add a testing mode that ensures host-only code compiles without nvcc.

See NVIDIA/libcudacxx#340 for where @wmaxey did the same thing in libcu++.