I downloaded the proxytunnel-1.7.2 source tarball from sf.net and was unable to compile using the provided makefile because no REV constant is defined in config.h.
I resolved it by adding '#define REV "2"' and the making of proxytunnel concluded without errors. REV is required by a printf function in file cmdline.c at line 38.
I hope this helps with the work on proxytunnel! Keep it up, it is really helpful.