general.m4sh: relative path to the same directory is `.'.

general.m4sh: relative path to the same directory is `.'.

* general.m4sh (func_relative_path): We sometimes want to use
the result of this function directly, rather than purely as a
prefix, so we need to return `.' when both directories end up at
the same location.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>