[CMake] Confusion with $<CONFIGURATION>

[Robert Dailey]

I'm using a custom target to copy files to the following directory:

${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/$<CONFIGURATION>

This works on Visual Studio generators, because binaries that are
compiled are placed in the directory above.

However, if I generate for NMake on Windows, binaries are placed in
${CMAKE_RUNTIME_OUTPUT_DIRECTORY} instead, but $<CONFIGURATION> for
the NMake makefiles generator says "Debug" instead of just "." so my
files do not copy to the correct location.

Can anyone explain this? Am I doing something wrong? How can I make my
files copy to the appropriate binary output directory on both
generators?