As the summary says, I'm attaching a patch that removes the -Werror flag, without it synce-rra builds on amd64 with gcc4.
Created attachment 80006 [details, diff] Removes -Werror flag from Makefile.in in lib/
and also when you try to build it with gcc4 in general (4.0.2-r3 in my case)
Having a build system force -Werror is a bad idea exactly because of these types of problems, so instead of your patch I added one that also removes -Werror from src/Makefile.*, to prevent this from recurring elsewhere with future compilers. Thanks for the report.