Build fixes for modern versions of binutils.

Need to explicitly list DSOs.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3091d7b..5ecd89e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -226,6 +226,9 @@
   if(X11_Xcursor_FOUND)
     set(FLTK_LIBRARIES ${FLTK_LIBRARIES} ${X11_Xcursor_LIB})
   endif()
+  if(X11_Xrender_FOUND)
+    set(FLTK_LIBRARIES ${FLTK_LIBRARIES} ${X11_Xrender_LIB})
+  endif()
 endif()
 
 if(FLTK_FOUND)