[Toybox] avoiding #ifdef

[enh]

thanks for changeset 1661:d4bc084916fd "id.c #ifdefectomy. (ifdefs
belong in headers, not in C code.)". at the time i wrote the change
that applied to i thought "i know the style is to avoid #if, but i'm
not sure what that's supposed to look like here", and now i have an
example. (in particular, it wasn't clear to me that it's okay to add
stuff to portability.h to support this style.)

thanks!

 --elliott