Gentoo's Bugzilla –
Attachment 395080 Details for
Bug 538100sys-libs/glibc-2.20-r1 with CC=clang - configure:9: checking whether clang -m64 -B/var/tmp/portage/sys-libs/glibc-2.20-r1/temp/bfd-linker compiles in -mx32 mode by default // configure:5156: error: These critical programs are missing or too old: gcc