Clarification of change in constant used for text-containing pasteboards across Mac OS versions.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@9989 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Stop relying on uname and instead ask autoconf what platform
we are building for. This makes cross compiling things a lot
easier.


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@9990 ea41ed52-d2ee-0310-a9c1-e6b18d33e121