scons: Compatibility with Scons development version string

This ensures Mesa3D build doesn't fail in this case as encountered when
bisecting Scons source code while regression testing
and when testing 3.0.5.a.2

Technical details:
Scons version string has consistently been in this format:
so these formulas should strip alpha/beta flags and return Scons version:

- as string - `'.'.join(SCons.__version__.split('.')[:3])`
- as tuple of integers - `tuple(map(int, SCons.__version__.split('.')[:3]))`

- v2: Fixed Scons version retrieval formulas as string and tuple of integers.
- v3: Fixed Scons version string format description.

Cc: "19.0" <>
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merge request