* common.mk: partially revert r40183, which breaks building on
  other than source directory. (its commit log also says the same
  thing, but such failure is not reproducible on my environment
  and the commit breaks build on my environment)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40239 b2dd03c8-39d4-4d8f-98ff-823fe69b080e