Avoid GNU Makefile-isms, let Exim continue to build on BSD.
Handle per-OS dynamic-module compilation flags.
+PP/02 Let /dev/null have normal permissions.
+ The 4.73 fixes were a little too stringent and complained about the
+ permissions on /dev/null. Exempt it from some checks.
+ Reported by Andreas M. Kirchwitz.
+
Exim version 4.73
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