Use std::unordered_{set,map,multimap} directly instead of choosing between several implementations, since 747526e requires -std=c++0x.

The CMake modules for finding hash* wasn't working for me (on Mac OS X 10.6.8 using gcc-4.2.1), probably because HAVE_HASH_* was cached from the first check which fails. But since 747526e requires -std=c++0x anyway (at least gcc-4.6 in my tests), it it much easier to delete this configuration and use std::unordered_{set,map,multimap} directly.