The posixmodule.c seems to have a lot of really complex CPP logic about
what chunks to compile.  If the target is to put the code there, I would
have to be guessing - next to the HAVE_GETEUID looks promising.

But wouldn't it be better to have the configure script check for the
presence of e.g. getresuid and compile it iff it finds it?  So that
sounds like some work with the autoconf script...