Build script

Fixed python modules update check. Always check if mako not found, ask otherwise.
Made build script aware of Mesa branches, helps with patches applicability narrowing.
Add Scons 3.0.0 compatibility patch to Mesa3D Git-powered auto-patches.
Only apply it to Mesa stable, patch is upstream now.
Determine Mesa branch even if it is not built. Preparation for S3TC merger.
Ensure auto-patching is done once and only once.
Look for Git before trying to patch. It is pointless if it is not found.
This script won't find Scons if it isn't in a certain relative location to it.
Addressed this by fixing scons locating when Python it's in PATH right from the beginning.
Added Scons to python update checking and auto-install. Depends on wheel.
1 less dependency requiring manual installation.
Halt execution if Mesa is missing and it can't be acquired or its acquiring is refused by user.
Drop LLVM 5.0 compatibility patch. Patch is upstream in all active branches.

Build script documentation

There is a compatibility fix for Scons 3.0.0.
Scons can now be acquired by build script automatically. Depends on wheel.
Git version control is now mandatory due to compatibility with latest Scons needing a patch.
Updated questions list asked by this script.

Mesa build: Scons 3.0.0 always uses target architecture compiler when using cross-compiling environment. This may impact compilation performance when making 32-bit builds.