Added command line option for Scons to select which version of MSVS to use. ...

Added command line option for Scons to select which version of MSVS to use.   Versions 7.1, 8.0 and 9.0 are allowed.   Usage is scons MSVS_VERSION=8.0.

SConstruct

@@ -50,6 +50,8 @@ opts.Add(ListOption('drivers', 'pipe drivers to build', default_drivers,
 opts.Add(ListOption('winsys', 'winsys drivers to build', default_winsys,
                      ['xlib', 'intel', 'gdi'])) 
 
+opts.Add(EnumOption('MSVS_VERSION', 'MS Visual C++ version', None, allowed_values=('7.1', '8.0', '9.0')))
+
 env = Environment(
 	options = opts,
 	tools = ['gallium'],