scons: Target Windows XP on userspace.
diff --git a/scons/gallium.py b/scons/gallium.py
index 59ec6ec..1136d26 100644
--- a/scons/gallium.py
+++ b/scons/gallium.py
@@ -221,6 +221,8 @@
             '_WINDOWS',
             '_UNICODE',
             'UNICODE',
+            ('_WIN32_WINNT', '0x0501'), # minimum required OS version
+            ('WINVER', '0x0501'),
             # http://msdn2.microsoft.com/en-us/library/6dwk3a1z.aspx,
             'WIN32_LEAN_AND_MEAN',
             'VC_EXTRALEAN',