don't run external_builder test on < VS2010

msvs_external_builder is MSBuild-specific and so fails on 2008. Fixes currently red win32/64 bots at http://build.chromium.org/f/client/gyp/.

R=bradnelson@google.com, sbaig1@bloomberg.net

Review URL: https://codereview.chromium.org/14198016

git-svn-id: http://gyp.googlecode.com/svn/trunk@1621 78cadc50-ecff-11dd-a971-7dbc132099af