Update makesrcdist script.

This script is used to generate source distributions. The update
includes adjustments for the new URL's since the server move, new
comments on how to use it, and a slightly changed behavior (see docs
in the file, points (2) and (3).


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10417 ea41ed52-d2ee-0310-a9c1-e6b18d33e121