How to Pass CMake variables into Wix fragments?

I’m currently struggeling with a custom CPackWiXPatch file where I’m referencing a bmp file for a dialog banner.
I can use a hardcoded path (for testing) which works, but I need to make this relative to CMAKE_SOURCE_DIR. It is easy in Wix to use variables like <Binary Id="MyBitmap" SourceFile="$(var.FOOBAR)/bitmap.bmp" /> but I cannot modify the generated cpack_variables.wxi file, which contains a fix set of variables. (This would be a nice feature, though).

I can create the file properties.wxi, which gets included by the main Wix project, but I don’t know how I should use this.

  1. The $(var) syntax requires a pre-processor define, not a property
  2. [SETUPDIR]\bitmap.bmp produces a file not found error. The message contains the literal [SETUPDIR] string, so this doesn’t get replaced
  3. "!(\bitmap.bmp" also doesn’t work.

How are these properties supposed to work?