This bug was solved before finishing the initial report, but is being filing anyway for posterity (in case someone else comes across it), as no other examples could be found. tl;dr: If you have the flag "-mabm" set in CFLAGS and/or CXXFLAGS in /etc/portage/make.conf, replace it with the flags "-mpopcnt -mlzcnt". Compiling net-print/cups would while running ./configure fail with the following: checking for x86_64-pc-linux-gnu-clang... x86_64-pc-linux-gnu-clang checking whether the C compiler works... no configure: error: in `/var/tmp/portage/net-print/cups-1.7.5/work/cups-1.7.5-abi_x86_64.amd64': configure: error: C compiler cannot create executables Trying to force cups to use gcc with 'CC="gcc" emerge cups' would allow the tests in ./configure to pass, but once it hit some .cxx files during the compile phase, it would give the following error: x86_64-pc-linux-gnu-clang-3.5.1: error: unknown argument: '-mabm' The inherent issue was "-mabm" was set for my CFLAGS and CXXFLAGS, which gcc understood but clang didn't. (see <http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20121112/068376.html> for more info) Instead of "-mabm", "-mpopcnt -mlzcnt" should be used, which is equivalent and understood by both gcc and clang.
