The fix proposed there is to include stddef.h in apr.h, in order to make sure that NULL is defined as a pointer type, such as (void *)0. Unfortunately, this doesn't work for all systems, since some (e.g. OpenBSD) define NULL as 0L, rather than (void *)0.
So in spite of regrets about churn, I whipped up a sed script that changed all places triggering the warning to use (char *)NULL instead of NULL as sentinel, and adjusted the result to fix overlong lines and other minor formatting issues. This fixes all current instances of the warning for me, making it much easier to spot important warnings. No functional change.
* subversion/mod_authz_svn/mod_authz_svn.c (get_access_conf, req_check_access, req_check_access): Use (char *)NULL as sentinel for apr_pstrcat(), instead of NULL.