Ant top-level target rework to allow single-platform use.  Not that whereas "ant build" used to imply, by dependence through -do, "dist", it is now just building (and the default target is changed to "dist").  For develepment work, consider "ant dist-dev" to build a single-platform distribution without docs or samples.  For single-platform work, consider "ant dist-one" (or "ant buildonly," which now does only one platform).

Review by: zundel, scottb

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5557 8db76d5a-ed1c-0410-87a9-c151d255dfc7