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>
21 jobs for !405 with scons-devel-compat in 127 minutes and 51 seconds (queued for 156 minutes and 47 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #165496
ubuntu

00:00:21

 
  Build+Test
passed #165510
make-gallium-drivers-other

00:11:02

passed #165509
make-gallium-drivers-radeonsi

00:07:16

passed #165508
make-gallium-drivers-swr

00:12:32

passed #165511
make-gallium-st-clover-llvm

00:06:41

passed #165512
make-gallium-st-clover-llvm-39

00:06:22

passed #165513
make-gallium-st-other

00:05:16

passed #165507
make-loader-classic-dri

00:10:14

passed #165506
make-vulkan

00:07:53

passed #165503
meson-gallium-clover-llvm

00:03:26

passed #165504
meson-gallium-clover-llvm39

00:03:39

passed #165502
meson-gallium-drivers-other

00:05:43

passed #165501
meson-gallium-radeonsi

00:03:05

passed #165505
meson-gallium-st-other

00:03:52

passed #165500
meson-gallium-swr

00:10:45

passed #165499
meson-glvnd

00:02:23

passed #165498
meson-loader-classic-dri

00:06:50

passed #165497
meson-vulkan

00:02:07

passed #165515
scons-llvm

00:05:13

passed #165514
scons-nollvm

00:05:16

passed #165516
scons-swr

00:07:45