waf: consolidate libintl related checks

waf: consolidate libintl related checks

consolidate the dealing with functions from libintl and the
handling of checking if libiconv is required or not
to a common place in lib/replace

also add a new samba_intl subsystem that has dependencies
on the appropriate set of libraries (libintl, libintl+libiconv or none)
that can be used as a general dependency by code that depends
on the internationalization libraries

Signed-off-by: Christian Ambach <ambi@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>