Support CMake versions 3.8-3.12.4 - #314

Thanks for working on support for older CMake versions, and sorry for the long wait. Making the build work on more toolchains is welcome, but I can't merge this PR as it stands. It contains two separate changes:

1. FindPythonLibs fallback for CMake < 3.12.4. I'm closing this part without prejudice. The fallback has some problems:

• FindPythonLibs doesn't accept COMPONENTS, so Interpreter Development NumPy is silently ignored. Nothing finds the numpy include directory, and WITHOUT_NUMPY isn't defined either. On most systems, numpy/arrayobject.h is not under the Python include directory, so the build would fail when it compiles the header.
• The package isn't REQUIRED, so a missing Python shows up later as a confusing compile error instead of a clear configure-time error.
• The version cutoff doesn't match what the else() branch needs. FindPython3 exists from CMake 3.12, but its NumPy component needs 3.14 and the Python3::Module target needs 3.15. CMake 3.12–3.14 would still fail.

If you'd like to try again, a resubmission is welcome. A version that picks the fallback based on the features actually used, finds the numpy include path (for example by asking the Python interpreter for numpy.get_include()) or else defines WITHOUT_NUMPY, and marks Python as REQUIRED would be a good starting point.

2. Reindentation of the rest of CMakeLists.txt. I'm closing this part as well. It's a whitespace-only change unrelated to the fix, and it makes the functional change harder to review. In any resubmission, please keep the existing 2-space indentation and limit the diff to the lines that need to change.

Thanks again for the contribution!

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