r489819 ("Unbreak port in FreeBSD 12 and 13-CURRENT") breaks the build for FreeBSD 11. This is because FreeBSD 11's sys/libkern.h has a prototype for memset() while later versions do not. Here's an updated bsddefs.h patch that undefines memset when set and always defines memset to oss_memset.
Created attachment 201080 [details] patch
A commit references this bug: Author: fernape Date: Sat Jan 12 23:04:06 UTC 2019 New revision: 490082 URL: https://svnweb.freebsd.org/changeset/ports/490082 Log: audio/oss: Fix breakage after r489819 The patch in r489819 broke the build in 11. Address that and make sure this one builds in {11.2,12}{amd64,i386} and 13-CURRENT i386 PR: 234909 Reported by: leres@ Changes: head/audio/oss/files/patch-kernel_OS_FreeBSD_wrapper_bsddefs.h head/audio/oss/files/patch-setup_srcconf__freebsd.inc
Committed similar patch. Thanks!