FindDoxygen: Fix setting of HAVE_DOT in non-backward-compat mode

`DOXYGEN_DOT_FOUND` is only set if `_Doxygen_keep_backward_compat` is
used (when no components are requested), so use `Doxygen_dot_FOUND`
directly.  Preserve the "YES" or "NO" value used previously.

Modules/FindDoxygen.cmake

@@ -832,7 +832,11 @@ doxygen_add_docs() for target ${targetName}")
         # and Lucent Bell Labs. The other options in this section have no
         # effect if this option is set to NO.
         # Doxygen's default value is: NO.
-        set(DOXYGEN_HAVE_DOT ${DOXYGEN_DOT_FOUND})
+        if(Doxygen_dot_FOUND)
+          set(DOXYGEN_HAVE_DOT "YES")
+        else()
+          set(DOXYGEN_HAVE_DOT "NO")
+        endif()
     endif()
 
     if(NOT DEFINED DOXYGEN_DOT_MULTI_TARGETS)