configure.in: use SSE2

* configure.in: use SSE2 instructions to drop unexpected precisions on
  other than mingw.  [ruby-core:59472] [Bug #8358]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44538 b2dd03c8-39d4-4d8f-98ff-823fe69b080e