Log Message:
m4: Improve macOS patch compatibility.

This should now work across all versions, and fix the build problem
raised in NetBSD/pkgsrc#158.  Tested locally against the 10.4 SDK.

From Jonathan Perkin <jperkin@smartos.org>
via drecklypkg commit 05d082ecd383bb9abfeebc219de0ff7ee50645c8