Build system's Fedora Core Development repository is out-of-date. According to http://extras64.linux.duke.edu/failed/development/sylpheed/2.0.0-0.3.beta4/i386/sylpheed-2.0.0-0.3.beta4.failure.log it installed cairo-devel-0.5.1-3 and caused the build to fail, whereas cairo-devel-0.5.1-4 from June 26th would have contained a fix.
Modified the mach configuration to point back at a regularly-updated mirror instead of the temporary mirror that was in use after the release.