`BASERUBY` may be not available when building from tarball

* common.mk ($(REVISION_H)): revert a part of r61055 because this obstructs
  building ruby from tarball in clean (no ruby) environment.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61308 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

common.mk

@@ -994,7 +994,7 @@ $(srcdir)/revision.h:
 	@exit > $@
 
 $(REVISION_H): $(srcdir)/version.h $(srcdir)/tool/file2lastrev.rb $(REVISION_FORCE)
-	$(Q) $(BASERUBY) $(srcdir)/tool/file2lastrev.rb -q --revision.h "$(srcdir)" > revision.tmp
+	-$(Q) $(BASERUBY) $(srcdir)/tool/file2lastrev.rb -q --revision.h "$(srcdir)" > revision.tmp
 	$(Q)$(IFCHANGE) "--timestamp=$@" "$(srcdir)/revision.h" revision.tmp
 
 $(srcdir)/ext/ripper/ripper.c: $(srcdir)/parse.y id.h