(asdf:operate 'asdf:compile-op :sdl)
succeeds ? The weird thing is that I see the same compilation of the same file except for a final:
;/path/to/cl-sdl/sdl/sdl.fasl written ; compilation finished in 0:00:00 WARNING: COMPILE-FILE warned while performing #
on #<cl-source-file sdl="">. ; ; compilation unit aborted ; caught 1 fatal ERROR condition ; caught 2 WARNING conditions ; printed 20 notes
I know the file compiles just fine. Hell, even when things go wrong it still says it wrote out the fasl. So it must be something with asdf. Unfortunately, I have no idea where to start looking...
 I read somewhere that "asdf regards a full warning as an error". There are indeed two warnings, so maybe that's what is tripping things up. Two options then: try to fix the warnings, or figure out how to change this behaviour in asdf...
 Here is a message I found stating the "warning is error" behaviour. It's really counterintuitive; which is to say that every other compiler I have ever used treats warnings just as warnings. Ah well. It seems the best option is to fix the warnings, so I'll start looking into that.