common.mk: no expansion

* common.mk (showconfig): get rid of variable expansion by shell.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39177 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-02-10

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39178 b2dd03c8-39d4-4d8f-98ff-823fe69b080e