Since commit 059c230d (VS: Explicitly default to v100 toolset in Visual
Studio 2010, 2016-07-21) the VS 2010 generator now correctly defaults to
the v100 toolset instead of no toolset.  However, this broke our logic
for defaulting to the `Windows7.1SDK` toolset for 64-bit builds on VS
2010 Express.  Fix the logic by ignoring the `v100` default in the case.