Use settings in wx_vcN_local.props files if they exist.

Allow overriding the default build settings in local properties files for VC11
and VC12 builds.

See #15780.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75982 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775