more el5 static build enhancements

Various fixes for linker issues when building fltk statically.

* The native FindX11.cmake module can't be overridden so that
  fltk links against the static libraries that we've just built.
* Need to link fltk against libdl.a or else an unresolved
  dependency error occurs while linking tigervnc.
* Additional fixes to fix dependency resolution issues when
  linking fltk against X11 statically.
3 files changed