mkmf.rb: prefer $LIBPATH than $DEFLIBPATH

* lib/mkmf.rb (link_command, libpathflag, create_makefile): prefer
  user specified `$LIBPATH` than `$DEFLIBPATH`.  [ruby-core:62100]
  [ruby-trunk - Bug #9760]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45640 b2dd03c8-39d4-4d8f-98ff-823fe69b080e