Extend Visual Studio sln generator to accept MAP_IMPORTED_CONFIG_*.
EDIT: This was accidentally merged and rewound. Replaced by !669 (merged).
This allows 3rd party projects added via include_external_msproject, to compile preferred configuration despite different naming standard
Inspired by old mail: https://cmake.org/pipermail/cmake/2010-April/036388.html
Topic-rename: include_external_msproject-map-config