xerces-c version has been pushed to 3.2.0, and old 3.1.4 .tar.gz package is not available on any mirrors.
confirmed: GENTOO_MIRRORS="" ebuild xerces-c-3.1.4-r1.ebuild fetch [...] 2017-12-19 03:14:23 ERROR 404: Not Found. !!! Couldn't download 'xerces-c-3.1.4.tar.gz'. Aborting. You should be able to install, if you setup the gentoo mirrors.