This is something Common Lisp does with its reader macros. There's a global variable FEATURES, which is populated by one's implementation (and so might indicate OS, implementation &c.). When one reads in a file of code, there're two reader macros #+ & #- which conditionally include code, based on whether or no symbols are present in FEATURES. E.g.:

    (defvar *foo* #+sbcl(sb-sys:thing) #-sbcl(portable-thing))

The elided code isn't even truly read in, so it's not an error to refer to the package SB-SYS in a non-SBCL implementation.

I wouldn't claim that this is the end-all, be-all of code, but it does make for some reasonably compact platform-dependent-but-portable code.