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.
So you focus on the header and do not try to check against the library. Otherwise I approve removing that test from the default place to the post processing area. I note that you put "[]" for the default, should it rather be "[with_ntl=yes]" or something of that effect?