version.c: show malloc_conf

* configure.in (jemalloc): check for the header regardless drop-in
  libjemalloc is found, for `malloc_conf` declaration.
* version.c (ruby_show_version): show `malloc_conf` if set.
  [Feature #9113]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46355 b2dd03c8-39d4-4d8f-98ff-823fe69b080e