cmake HAVE_HASH_FUNC_HEADER examples

datastax/cpp-driver src/third_party/sparsehash/CMakeLists.txt :75 

unset(HAVE_HASH_FUNC_HEADER CACHE)

datastax/cpp-driver src/third_party/sparsehash/CMakeLists.txt :76 

check_cxx_source_compiles(
    "
    #include <${LOCATION}>
    int main() { int x = ${HASH_NAMESPACE}::hash<int>()(42); }
    "
    HAVE_HASH_FUNC_HEADER)