defs/gmake.mk: no _FORTIFY_SOURCE unless optimization

* defs/gmake.mk (XCFLAGS): remove _FORTIFY_SOURCE if optimization is
  disabled as it requires compiling with optimization.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45402 b2dd03c8-39d4-4d8f-98ff-823fe69b080e