GenExp $<CONFIG> in linked library target sources fails using Unix Makefiles

The generator expression $<CONFIG> is not evaluated to CMAKE_BUILD_TYPE when used in the sources of a library target that is linked by another target.

Small example project: cmakeConfigLibBug.tar.gz

CMakeLists.txt from the example

cmake_minimum_required ( VERSION 3.7.2 )

# Set build type
set(CMAKE_BUILD_TYPE Debug)

# Library with $<CONFIG> fails
add_library(libA STATIC libA_$<CONFIG>.cpp)

# Executable that links to library
add_executable ( binA main.cpp )
target_link_libraries(binA libA)

This is a roadblock for !858 (merged)