How to make files from a primary and secondary dir

[Joakim Ekström]

I have a complete set of source files in a directory called "orig".
Another version of the source is placed in the "new" directory,
but only files that differs from "orig" are placed in "new".

 /new/Makefile
 /new/foo.c (may or may not be present)
 /new/foo.h (may or may not be present)

 /orig/foo.c
 /orig/foo.h

I would like to have a Makefile that primarily
takes the files in the "new" directory, if present.
If a file is not present in "new",
the file in "orig" should be used instead.

How should a Makefile be written to build the source in "new"?

(I checked out http://www.gnu.org/software/make/manual/make.html
and have tried with VPATH and -I , but I cant seem to get it to work.)

Regards
Joakim