A VPATH build of diffutils (git HEAD, with GNU make, on Ubuntu 22.04)
fails like this:

$ mkdir bb
$ cd bb
$ ../configure
$ make
...
make[1]: Entering directory '/DIFFUTILS/diffutils/bb/src'
  GEN      paths.h
  CC       cmp.o
In file included from ../../src/cmp.c:19:
../../src/system.h:87:10: fatal error: version.h: No such file or directory
   87 | #include "version.h"
      |          ^~~~~~~~~~~
compilation terminated.
make[1]: *** [Makefile:2451: cmp.o] Error 1


The attached patch fixes it.