Try to detect NTL more reliably

Ok, this patch does what we discussed today: it will refuse to compile when --without-ntl is given, and whether --with-ntl is given or not, it will try to check the correctness of the flags.

It doesn't do a very thorough check, but I can add more if you think it's worth it.
As I mentioned in the pull request, I only do a basic check. I could also try to do like linbox and compile&run a small test program... but is it worth the effort?

I don't think I need with-ntl=yes since the 'case' below checks both for yes and nothing and in that case puts default values in the NTL_* variables.