configure: improve api/ detection

API was never selected by default, because <stdc++> should have been
<bits/stdc++> and in any case needs g++.  Boost dependency has recently
been lifted, but we added gnu++11 dependency.
Only x86 currently uses compiles api, so make the room for errors a bit
smaller by checking for it.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>