Change SONAME on OSX

It stated that the soname file could be deleted, but that wasn't
actually the case. Now we put a real name in the SONAME, which is not
really a SONAME on OSX, that's too bad.
Force clang on OSX

From #travis@irc.freenode.net, it seems gcc points to clang anyway on
OSX now, so just force it in the makefile. This has the advantage of
putting the right options in the right variables (`-Qunused-arguments'
in WFLAGS for instance).