On an Oracle 12.4 build the c_using_fortran executable cannot find the
"fsu" library at runtime. Since this is an implementation detail of the
"hello" library, link that library to it privately so that "-lfsu" does
not propagate to the executables consuming it.
cmake_add_fortran_directory uses imported targets when using the
mingw fortran compiler. This change makes those targets global
in scope so they act just like the real targets that exist when
a fortran compiler exists and regular add_subdirectory is used.