Fix prerequisite handling when there are none

Fix prerequisite handling when there are none

In a directory with no sources, there won’t be any metadata, and so
the `$(and …)` in `$(diffprereqs)` would exit early, before creating
the file in `$(BUILDDIR)/lastprereqs`. However, this latter file is
still returned as a prerequisite. This commit touches it if it doesn’t
exist to ensure it is always created.