updated FLTK: add --universal option, build with --enable-shared

This commit adds two things:

1) option to build with --universal
2) adds --enable-shared when running configure

--enable-shared gets the build to generate shared libs (dylibs).  The project I work on that uses FLTK created multiple plugin libraries that use FLTK.  When compiling statically, those plugins will have symbol conflicts between the various FLTK functions that were statically included.  Using shared libs fixes this situation.