As far as I can tell, OpenLDAP has been unusable with K&R C since openldap-2.0: lber.h in that release had prototypes unprotected by LDAP_P() in struct sockbuf_io. They've been there ever since. So LDAP_P() can go.
Don't know which other not-strict-ISO support is still useful - I note lber.h has used <string.h> since openldap-2.2.4, for example. libldap/open.c has used <limits.h> since 2.0.0. HEAD wraps that in HAVE_LIMITS_H, but it does not protect INT_MAX the same way.