scons: Warn when using MSVS versions prior to 2012.

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

scons/gallium.py

@@ -402,6 +402,8 @@ def generate(env):
               '/Oi', # enable intrinsic functions
             ]
         else:
+            if distutils.version.LooseVersion(env['MSVC_VERSION']) < distutils.version.LooseVersion('11.0'):
+                print 'scons: warning: Visual Studio versions prior to 2012 are known to produce incorrect code when optimizations are enabled ( https://bugs.freedesktop.org/show_bug.cgi?id=58718 )'
             ccflags += [
                 '/O2', # optimize for speed
             ]