[PATCH] Fix Makefile.macosx for users without Xcode

[josh]

Hello there, hope you're all well.

I recently ran into trouble building Chicken on my MacOS system (Big Sur
11.2.3) due to issues with command-line tool paths. There was a change in
c6a6a26225620b566f8bc487cc4180de55555a2a that assumed the user has Xcode
installed, and that the tools are within /Applications/Xcode.app/.

However, it's possible to install command-line tools on MacOS without Xcode
(if one tries to run e.g. `gcc` on a fresh MacOS install you'll be prompted
to do so), in which case the Makefile's assumptions are not sufficient. This
patch adds a check to see if Xcode is installed, and if not uses the tools
in /usr/bin instead.

(Please note that this is my first patch to Chicken, and first patch in
general :P, so my apologies if I'm missing anything. I'm also not familiar
with GNU's make so I hope the construct I chose is portable enough.)

Thank you all for your time and hard work!
Josh

macos_tools_path.patch
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