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.
We do not yet support "make install" in the external project case.
Document this explicitly in the interface. Require the caller to use an
option to "disable" the unsupported behavior. This will allow us to add
the behavior by default in the future without clobbering existing
projects that handle the installation themselves.
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.
