* bignum.c: Use GMP to accelerate big Bignum multiplication.
  (bary_mul_gmp): New function.
  (bary_mul): Use bary_mul_gmp.
  (bigsq): Use different threshold with GMP.

* configure.in: Detect GMP.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42743 b2dd03c8-39d4-4d8f-98ff-823fe69b080e