Distribute version.mk in case autoconf needs to be run again

Users may need to rebuild the configure script, so we need to distribute
version.mk so autoconf can run successfully.
diff --git a/Makefile b/Makefile
index 47c8eac..0fa309b 100644
--- a/Makefile
+++ b/Makefile
@@ -192,6 +192,7 @@
 	$(DIRECTORY)/bin/install-sh					\
 	$(DIRECTORY)/bin/mklib						\
 	$(DIRECTORY)/bin/minstall					\
+	$(DIRECTORY)/bin/version.mk					\
 	$(DIRECTORY)/configs/[a-z]*					\
 	$(DIRECTORY)/docs/*.html					\
 	$(DIRECTORY)/docs/COPYING					\