Make gettext use generator, not main target class.

* tools/gettext.jam: The above change.
* build/generators.jam (construct): Allow a generator to produce a target
  without type.


[SVN r26103]

tools/build

@@ -1 +1 @@
-Subproject commit 14ce4c5cac331a8693bbc56c86099f11551a9011
+Subproject commit 1ccdad1403ab7234d4684026fd2f9eb84cba3abc