[Bug-apl] gnu-apl-mode: minor M-. issue

[David B. Lamkins]

This one probably doesn't affect many people.

If you define a function using ⎕FX without using the GNU APL extension
to store location metadata, a M-. on that function's name will complain:

if: Unexpected tag format: "⎕FX"

It might be nice if this said something like:

"This function was defined using ⎕FX; location information is
unavailable."