Only link with fontconfig if Xft is used

This matches what the autoconf stuff does. It is also crucial
on OS X where linking against fontconfig can fool the system into
believing that FLTK requires X11, prompting the installation of
XQuartz.


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10244 ea41ed52-d2ee-0310-a9c1-e6b18d33e121