To make it short: gnupg won't be able to encrypt messages, when you compile it with the CFLAG -fwrapv. So IMHO it would be wise to block that flag for gnupg. There was another user who had the same problem: http://forums.gentoo.org/viewtopic-t-537426-highlight-gnupg.html?sid=f1b5f22e652773daf64b41e5b4ef27de
Sorry, we won't filter every single whacky compiler flag that someone decided to use for whatever reason; sorry.