In commit d440747aaefc61bdd19e71e09ab4f28aa93795d7, README.volumes was removed but there is still a reference to it in docs/Makefile.am that throws an error when building Thunar. The same for ThumbnailersCacheFormat.txt that was removed in commit d09891367404fc53c9ec79eaa9838ee96565737e.
Oops, this has long been fixed in master: commit 5e554f8be243bd9691c0345ef0eb1b09c0e90a31 Author: Jannis Pohlmann <jannis@xfce.org> Date: Tue Aug 31 15:16:31 2010 +0200 Remove files deleted in the last commit from the Makefile (bug #6676).