FindDoxygen: Restore DOXYGEN_DOT_FOUND variable

The changes in commit v3.9.0-rc1~55^2 (Improve Doxygen support,
2017-04-10) accidentally dropped this variable from its compatibility
mode.

Modules/FindDoxygen.cmake

@@ -586,7 +586,7 @@ else()
     set(DOXYGEN_FOUND "NO")
 endif()
 if(_Doxygen_keep_backward_compat)
-    if(DOXYGEN_DOT_FOUND)
+    if(Doxygen_dot_FOUND)
         set(DOXYGEN_DOT_FOUND "YES")
     else()
         set(DOXYGEN_DOT_FOUND "NO")