bug#59406: [Update install instruction for macOS] Update install instruction for macOS

[Karl Berry]

    --- a/INSTALL
    +++ b/INSTALL
    +On macOS, users should not use the default perl, but manually install
    +it from https://www.perl.org/get.html.

Thanks for the suggestion. I have a couple comments:

1) INSTALL is maintained in Autoconf, not Automake. I'll try to redirect
the bug.

2) Why? I know plenty of people using the mac perl ok for various things.
It is a big barrier to install a new perl. Not something to recommend
lightly.

3) Even if needed, in general, I feel doubtful that such a blanket and
time-sensitive statement should be made in INSTALL. I see INSTALL still
contains a couple of notes on "Particular systems", though two are
hardly used any more as far as I know (HP-UX and OSF/1), and the other
tidbits feel pretty old also. As a matter of practical usefulness, I
feel like INSTALL is better off being about system-independent
configuration, leaving system information for web pages or other places
that aren't so static. But it's not up to me.

4) Notwithstanding all the above, at the very least, I think any such
statement should say something specific about how the default perl fails.

Anyway, passing over to autoconf ... --best, karl.