configure.in: libruby needs gmp

* configure.in (SOLIBS): LIBRUBY_SO also needs linking with gmp, to
  run worker processes in test-all on non-ELF platforms.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42817 b2dd03c8-39d4-4d8f-98ff-823fe69b080e