Created attachment 3270
Xfce4-mailwatch-plugin uses some functions from libgcrypt, without explicitly linking against it, which makes the linking fails with stricter linkers such as binutils-gold.
The attached patch works for me (I'm not an autotools guru, so it can surely be improved). FWIW, I generated it after applying the 4.7 panel patch described in Bug #6608.
Created attachment 3271
(the previous version had some spurious headers, sorry)