Consider the case motivating commit e01cce28 (Allow add_dependencies()
on imported targets, 2010-11-19).  An imported target references a file
generated at build time by a custom target on which it depends.  Had the
file been built directly using add_library or add_executable its target
name would have been visible globally.  Therefore the imported target
representing the file should be globally visible also.

Teach the IMPORTED signature of add_(executable|library) to accept a new
"GLOBAL" option to make the imported target visible globally.