Externally Controlled CMake Binaries

[ax3l]

Hi,

for package managers like Spack (https://github.com/spack/spack/pull/28760), we are able to control the CMake source exactly and need to be able to work on air-gapped machines.

We have already a cmake copy installed and would like to use that when building py-cmake. The reason for that is that this allows packagers to maintain system- and compiler-specific patches and work-arounds for building cmake itself in one location. The risk of version mismatch between py-cmake and cmake is mitigated in such an environment, because we control the exact versions in Spack.

Currently, we cannot point to an existing CMake binary install: we can only point to CMake sources or download CMake binaries.