_MSC_VERs 1400 and greater require manifests. Check against

_MSC_VERs 1400 and greater require manifests. Check against
MSC_VER_REQUIRED when we run mt.exe instead of checking for each
individual MSVC_VARIANT. This fixes the current buildbot test failures
on Windows, which resulted from a missing check for MSVC2008. This
also keeps us from having to mess with a bunch of makefiles when we add
support for new Visual C++ versions.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@26052 f5534014-38df-0310-8fa8-9805f1628bb7