It's not install() friendly, so we use $<CONFIG> instead in
some cases. Unfortunately, $<CONFIG> isn't friendly with
add_custom_command's OUTPUT options while CMAKE_CFG_INTDIR
explicitly is, so we use a hybrid approach until CMake gets
its act together.
This changes the structure to use paths like `build/Release/lib`
instead of `build/lib/Release`, so that the `build/Release` directory
more closely mimics the structure of `build` when in single-configuration
mode.