Version 5 only ships with automake 2.59, but gnuplot now requires at least 2.60 to build (CVS). This patch includes the necessary changes for this old, but still supported platform.
I don't think that's correct. The requirement is for autoconf 2.60 (not automake). And this is needed only by the ./prepare script. A distribution package made from the current CVS should build just fine, since it doesn't require autoconf at all.
As to the APPLE/OBJC macros, aren't they necessary for building on OSX? (I'm not sure).
You are correct about autoconf/automake. But this patch is only meant to help anyone who wants to build current CVS on SL5 e.g. (like me). It is not meant for inclusion in CVS. So anything Mac related is certainly irrelevant.