Enable CUDA in CMakeList.txt for Spack smoke test.

[etpalmer63]

Summary

Adds the necessary lines to CMakeLists.txt to enable CMake to compile the Spack smoke test with CUDA.

Additional background

Tested on spack/develop. Passed, but requires an extra step to make sure the CUDA compilers match. Next step is to adjust the smoke test script for AMReX in Spack.

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[sayerhs]

The code changes look fine. Would it be possible to link the spack script to this PR for future reference? In particular, it would be nice to see what arguments are passed to CMake via spack.

[WeiqunZhang]

https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/amrex/package.py