configure.in: -g and -fstack-protector flags can't work

 together on SmartOS
* configure.in (stack_protector): control use of -fstack-protector.
* configure.in (debugflags): let -fstack-protector precede and disable
  debugflags, because they can't work together on SmartOS.  [Bug #8268]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40364 b2dd03c8-39d4-4d8f-98ff-823fe69b080e