Add support for Sun/Uniforum/GNU gettext libintl. This belongs in CMake
rather than upstream because:
* There are multiple upstreams (Sun, GNU).
* It may or may not be in the glibc C library depending
upon the platform and build options used.
Although we already have a FindGettext module, that is for the tools.
This module is for the library, and is independent because it's
perfectly OK to use libintl without the gettext tools (and vice versa),
and they might not all be found. Add cross references between the two
modules in notes to make the relationship clearer.
Roger Leigh