Gentoo's env-update sets CC=gcc in the environment. Be sure to 'unset CC' before you build Unslung. If you've already had a build problem, do 'unset CC; make distclean; make' to clear everything out and redo the configurations.
If you have an older build on your Gentoo system, this can catch you by surprise. Most of the packages' configure routines ignore the CC in the environment, but a few newer ones (miau and sudo, for example) get confused and b0rk their configurations.
Note: Master Makefile v1.52 fixes this problem.