Reported by Lapo Luchini, Jan 17, 2014

Compiling with clang leads to many errors, and thus it's impossible 
to build on FreeBSD 10.

In file included from src/monotone.cc:22:
In file included from src/database.hh:19:
In file included from src/rev_types.hh:22:
In file included from src/hybrid_map.hh:14:
src/hash_map.hh:101:25: error: no member named '__stl_hash_string' 
in namespace '__gnu_cxx'
      return __gnu_cxx::__stl_hash_string(s.c_str());