Fix check for ABI-broken GCC 4.7.{0,1} libstdc++

...to avoid mis-clasification of e.g. GCC 4.6 libstdc++ on Ubuntu 12.04 as
witnessed at <http://ci.libreoffice.org/job/buildbot/2461/consoleFull>

Change-Id: Ia754a8d847e7af4e0e25b4bf82e17214f7212894