Removing virtual/x11 from DEPEND in the ebuild (with the customary changes in the MD5 and size in the Manifest) allowed for the compilation. Used bug #120934 as a template for entering this one. http://bugs.gentoo.org/show_bug.cgi?id=120934
It needs more than dropping virtual/x11, as your system may have extra libraries that are needed for compilation. See http://www.gentoo.org/proj/en/desktop/x/x11/porting-modular-x-howto.xml if you want more infos on how to find the real deps. Anyway, flpsed now has proper modular X dependencies in CVS.