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.
You are right, it will just work. I am in two minds on whether to check the library is present or not. a.k.a. check your install is not completely broken. I guess we should reserve ac_check_lib to check for a symbol that we know is needed or indicate a minimum version required (i.e. not present in older version). Of course you were probably thinking of the macro that takes header and library to try to compile and run.