common.mk: rebuild verconf.h for each configure

* win32/Makefile.sub (verconf.h): so depends on verconf.mk, which
  is rebuilt by setup.mak.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48664 b2dd03c8-39d4-4d8f-98ff-823fe69b080e