I cannot get the library ${static_library} to statically link using CMake. I have tried hard coding the path and as far as I'm aware TARGET_LINK_LIBRARIES should link this in automatically.
ADD_LIBRARY(libraryA STATIC ${source_files})
TARGET_LINK_LIBRARIES(libraryA debug ${static_library})
There is a sub project that links libraryA later in the build
ADD_EXECUTABLE(testA ${test_sources})
TARGET_LINK_LIBRARIES(testA libraryA)
which has ${static_library} correctly linked in but I need ${static_library} to be directly linked into libraryA.
You cannot link libraries into libraries directly.
Some compilers allow you to do this (e.g. MS Visual Studio has an option), but others don't, which is why CMake also won't allow it. See this answer for what your options are to deal with this.