Merge branch 'jc/unit-tests-make-relative-fix'

The mechanism to report the filename in the source code, used by
the unit-test machinery, assumed that the compiler expanded __FILE__
to the path to the source given to the $(CC), but some compilers
give full path, breaking the output.  This has been corrected.

* jc/unit-tests-make-relative-fix:
  unit-tests: do show relative file paths on non-Windows, too