CMake - don't require Python just to build

[idigdoug]

The BUILD.md document implies that re2c should be able to build without Python, but the CMake build has a hard requirement for Python. Fix the CMake build to match what BUILD.md says.

• Move the initialization of the PYTHON variable up higher in the script so it is available sooner.
• Only set the PYTHON variable if we find a suitable version of Python.
• If we can't find Python, it's only an error if we are rebuilding docs.
• If we can't find Python, don't add the test or check targets.

[idigdoug]

Makes sense to me. I'll update the BUILD.md pr accordingly.