fix(setup): support overriding CMake args

[henryiii]

Description

Supports setting CMAKE_ARGS to control the CMake file generation. Needed for conda-forge, which does not find the compiler otherwise.

See conda-forge/pybind11-feedstock#74.

Suggested changelog entry:

* Allow CMAKE_ARGS to override CMake args in pybind11's own setup.py.

[Skylion007]

We do this for Habitat-Sim as well. Surprised this issue wasn't already brought up and fixed.

[henryiii]

(On hold until I work out the actual problem with conda-forge - I think they are going 2017 -> 2019 and the transition is broken). If the original code works after this is sorted, I'd be up in the air about putting this in - we might want the ability to override the CMake command, so I think I'd still be in favor of it.

[henryiii]

Surprised this issue wasn't already brought up and fixed.

Building from true source is very rare - honestly, I probably could move conda-forge over to using the SDist, and not bother with the build from git source. However, I think the reason I don't do that is then I'd have two source files and it would be harder to automate the maintenance.

[henryiii]

This sort of patch is not needed to support conda-forge (see conda-forge/pybind11-feedstock#75). It might still be a good idea, though?