VS: Do not default to missing v100 64-bit toolset on VS 2010 Express

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.

Source/cmGlobalVisualStudio10Generator.cxx

@@ -461,6 +461,10 @@ void cmGlobalVisualStudio10Generator::GenerateBuildCommand(
 
 bool cmGlobalVisualStudio10Generator::Find64BitTools(cmMakefile* mf)
 {
+  if (this->DefaultPlatformToolset == "v100") {
+    // The v100 64-bit toolset does not exist in the express edition.
+    this->DefaultPlatformToolset.clear();
+  }
   if (this->GetPlatformToolset()) {
     return true;
   }