Option to add/affect cmake options generated with install.py

[fglines-nv]

There should be an option to affect the cmake build of cupynumeric (and the dependencies it builds). Things such as adding extra CMake flags, specifying CMake flags such as CMAKE_CXX_COMPILER, CMAKE_CXX_FLAGS, etc. If that capability already exists with install.py or with environment variables, then we should document it

[manopapad]

This might be a non-issue after https://github.com/nv-legate/cupynumeric.internal/pull/716 is merged, which drops install.py in favor of porting the build system from Legate.

[fglines-nv]

Agreed, the Legate build system is more flexible.

[cryos]

Many things can just be passed as environment variables, such as CXX and CC specifying the C++ and C compilers respectively. From my perspective the Python CMake argument generator can make things tougher and more custom/unique to that project. You can also export CFLAGS, CXXFLAGS, CUDAFLAGS see https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS.html where quite a few variables use standard environment variables supported by CMake and other build systems.