scons: Workaround failures with MSVC when using SCons 3.0.[2-4].

scons: Workaround failures with MSVC when using SCons 3.0.[2-4].

This change applies the workaround suggested by Bill Deegan on the
affected SCons versions.

It also adds a comment with the URL explaining why we were using
customizing the decider and max_drift in the first place, as I had
forgotten all about it.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109443
Tested-by: liviuprodea@yahoo.com
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>