Fix generate-changelog rule when building from outside of the source repository

Fix generate-changelog rule when building from outside of the source repository

The "make dist" target depends upon a phony target "generate-changelog" which
generates the ChangeLog file.  Unfortunately, this was failing if the current
directory was not the source directory.  Worse, this failure was not detected,
but would erroneously generate an empty ChangeLog.

This change fixes both these problems.