Support or explicitly reject older MCL

[eschaton]

On a lark, I tried to load Quicklisp on MCL 5.2 on an iBook G4 running Mac OS X 10.4.11. It failed because on that platform, there’s no CCL:make-socket to re-export from qlqs-ccl.

It’d be awesome if Quicklisp could support old versions of MCL, of course, but in the absence of support it should probably explicitly reject them.

[quicklisp]

What *features* can be used to distinguish Clozure CL from MCL?

[eschaton]

There’s at least :clozure. And it looks like only MCL has :mcl in it.