to build an X11-using version of FLTK on the Mac OS platform.
This matches the recent support of the --enable-x11 option
by the configure script on the mac.
git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@10986 ea41ed52-d2ee-0310-a9c1-e6b18d33e121