Clear `CMAKE_SHARED_LIBRARY_CREATE_<LANG>_FLAGS` since no special
link flags are needed to create a DLL with these toolchains.
Our shared library creation rules do not reference the placeholder,
so this variable was never used. However, we may soon teach the
generators to look it up directly.
This variable define how the link step is done. Possible values are:
* DRIVER: the compiler is used as driver for the link step
* LINKER: the linker is used directly for the link step.