Due to a change in toolchain-funcs.eclass, gcc-version now queries the active compiler instead of the active GCC. This causes a failure with CC="clang" as clang reports itself as GCC 4.2. Please see the tracker bug for more information.
Created attachment 440526 [details, diff] patch
Fixed by https://github.com/gentoo/gentoo/commit/d68b3eb31e7a76f8f94be296fc6c5fa61dc31fcb