Compile with Clang.

This bug surfaced after we switched Mac over to use
clang's libstdc++, which has __builtin_popcount but
does not mark it as constexpr.

Change-Id: I4260af48b00c6db3322e52fb113075d305b1e1ec
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>