CPack creates cpack_variables.wxi in the build directory. In the WiX
template it can be used by <?include "cpack_variables.wxi"?> 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
<?include "$(sys.CURRENTDIR)_CPack_Packages\win32\WIX\cpack_variables.wxi"?>.
This requires knowledge about the build directory structure and
is avoided by this change by adding the build directory to the IncludeSearchPaths.