In findlib's default configuration `metadir` is empty, so the
"already installed" check resolves `META.<pkg>` against the current
working directory. v1.6 renamed `META.legacy` to `META.num`, which is
exactly the file the check trips on: `ocamlfind install num META ...`
fails with "Package num is already installed (file META.num already
exists)" even in a clean destroot. Move `META.num` in place of `META`
so the source file is not present during install.

--- src/Makefile.orig
+++ src/Makefile
@@ -175,7 +175,7 @@

 findlib-install: num-top-install
-	cp META.num META
+	mv META.num META
 	$(OCAMLFIND) install num META $(TOINSTALL) $(TOINSTALL_CMXS) $(TOINSTALL_STUBS)
 	rm -f META

 num-top-uninstall: