scons: Target Windows XP on userspace.

scons/gallium.py

@@ -221,6 +221,8 @@ def generate(env):
             '_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',