Fall back to c_rehash if 'openssl rehash' fails

'openssl rehash' isn't implemented on all platforms, and since 'make
test' depends on a rehash of certs/demo being performed, it becomes an
effective block from running tests on any platform but Unix, for the
moment.

It's better to fall back to c_rehash and let the tests perform
everywhere.

Reviewed-by: Matt Caswell <matt@openssl.org>