SCons MSVC support relies on vcvarsall.bat to extract the PATH, CPP
includes, library paths, etc.

And SCons also has an build env var named MSVC_USE_SCRIPT which one can
use to point to alternative vcvarsall.bat script.

This change exposes this MSVC_USE_SCRIPT build env variable as a SCons
command line variable.  This will enable using MSVC outside Program
Files (e.g, network shares, etc.)

This change also links advapi32 library, necessary for the Windows
Registry API used by WGL state tracker, avoiding missing symbols.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>