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 https://bugs.freedesktop.org/show_bug.cgi?id=109443 and when testing 3.0.5.a.2

Technical details: Scons version string has consistently been in this format: MajorVersion.MinorVersion.Patch[.alpha/beta.yyyymmdd] 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" mesa-stable@lists.freedesktop.org