Libintl is actually the runtime portion of the gettext package
The Source is From: http://ftp.gnu.org/pub/gnu/gettext/
For PHP suitable sources and up-to-date information see https://github.com/winlibs/gettext.
The latest version includes locking to help with threading issues (finally)
PHP only builds the runtime libs, since mingw builds of the utilities work just fine to create and manipulate gettext files
The source required quite a few changes though, including a forgotten IO.h include for windows, some posix to ansi renames an inline to make wmemcpy cooperate on vc6 (7, and 7.1), and export fixes - diff should be pushed upstream at some point