VS: Explicitly default to v100 toolset in Visual Studio 2010

Otherwise `CMAKE_VS_PLATFORM_TOOLSET` is not set even though the `v100`
toolset is chosen.

Source/cmGlobalVisualStudio10Generator.cxx

@@ -101,6 +101,7 @@ cmGlobalVisualStudio10Generator::cmGlobalVisualStudio10Generator(
   this->SystemIsWindowsPhone = false;
   this->SystemIsWindowsStore = false;
   this->MSBuildCommandInitialized = false;
+  this->DefaultPlatformToolset = "v100";
   this->Version = VS10;
 }