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.