cmake HAVE_HASH_MAP examples

datastax/cpp-driver src/third_party/sparsehash/CMakeLists.txt :51 

unset(HAVE_HASH_MAP CACHE)

datastax/cpp-driver src/third_party/sparsehash/CMakeLists.txt :52 

check_cxx_source_compiles(
      "
      #include <${LOCATION}>
      int main() {
        ${NAMESPACE}::unordered_map<int,int> m;
        return m.find(42) == m.end();
      }
      "
      HAVE_HASH_MAP)