WiX: Add the build directory to IncludeSearchPaths for candle.exe
CPack creates cpack_variables.wxi in the build directory. In the WiX template it can be used by because the template is configured into main.wxs in the build directory. Because the extra source files are in the source directory it was necessary to use
.This requires knowledge about the build directory structure and is avoided by this change by adding the build directory to the IncludeSearchPaths.
Topic-rename: cpack-wix-extra-sources