scons: Don't restrict MSVC_VERSION values.

Saves the trouble of continuously needing to update.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
diff --git a/common.py b/common.py
index d6e6215..1d2d586 100644
--- a/common.py
+++ b/common.py
@@ -101,4 +101,4 @@
 	opts.Add(BoolOption('quiet', 'DEPRECATED: profile build', 'yes'))
 	opts.Add(BoolOption('texture_float', 'enable floating-point textures and renderbuffers', 'no'))
 	if host_platform == 'windows':
-		opts.Add(EnumOption('MSVC_VERSION', 'MS Visual C++ version', None, allowed_values=('7.1', '8.0', '9.0', '10.0', '11.0', '12.0')))
+		opts.Add('MSVC_VERSION', 'Microsoft Visual C/C++ version')