Same as most other languages.
You write portable CL first.
Write compatibility layers (if you can't find any) for other implementations if you need unportable features.
If you're implementing something with the FFI, again you should be aware of what you're doing that is platform specific.
CL code is by default portable, unless you decide to write non-portable code.
If in C you used
ifdef's, in CL you use conditional reader macros (which offer a lot more flexibility).
Pretty much all my code is portable, even the low-level one, however I do find myself limited to a handful of implementations if I'm writing truly non-portable stuff that messes with compiler's internals or worse. This being because I only implement the new features for the implementations I use (SBCL, ClozureCL, ECL and CLISP).
>>2
Out.