In order to debug a package as a normal user, emerge it like that:
$ FEATURES='nostrip' CFLAGS='-ggdb3' ebuild /usr/portage/media-sound/xmms/xmms-1.2.10-r15.ebuild unpack setup compile $ sudo ebuild /usr/portage/media-sound/xmms/xmms-1.2.10-r15.ebuildinstall qmerge