As of r399238, when the heimdal port option was selected, this port did
not build properly. Prior to r399238, even if the heimdal port was
installed, the base krb5 libraries and include files were used. This is
because ports/security/heimdal places its libraries in
${LOCALBASE}/lib/heimdal and include files in ${LOCALBASE}/include/heimdal,
which this port does not look for (unless told to do so... by this commit).

Discovered by: marino