firerfox and chromium requires clang-built dependencies

[Cuong Nguyen]

Tried building latest firefox [+clang] and chromium with clang-7. Both failed at the very end after couples of hours in linking stages with "Undefined reference to..." so I rebuilt those referencies-to library with clang-7, and ran ebuild <...> compile. Compile successfully completed.

Is it OK to have both clang and gcc in the same box?

Thank you

[Juippisi]