common.mk, Makefile.sub: fix build failure on mswin

* common.mk (verconf.h): $< cannot be used in explicit rules with
  nmake.
* win32/Makefile.sub (CONFIG_H): create verconf.in instead of
  verconf.h.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40770 b2dd03c8-39d4-4d8f-98ff-823fe69b080e