The commit "Consider link dependencies for link language" taught CMake
to propagate linker language preference from languages compiled into
libraries linked by a target.  It turns out this should only be done for
some languages, such as C++, because normally the language of the
program entry point (main) should be used.

We introduce variable CMAKE_<LANG>_LINKER_PREFERENCE_PROPAGATES to tell
CMake whether a language should propagate its linker preference across
targets.  Currently it is true only for C++.