[exim] Hint for build farmers of "pony" (krot) and "boar" (u…

Hi,

a recent change in the dmarc.c makes your animals failing the DMARC
checks for tests using HEAD (default branch "master").

If you link against the 1.3.x libopendmarc, you need an additional
Local/Makefile option "DMARC_API=100300" (see the "src/EDITME" file).

This should enable the "legacy" function call. The default is now to use
the 1.4.x API. Unfortunately the OpenDMARC project doesn't provide a
suitable macro/definition for easy autodetection.

References:
- Exim Bug 2728: https://bugs.exim.org/show_bug.cgi?id=2728
- OpenDMARC Issue 167: https://github.com/trusteddomainproject/OpenDMARC/issues/167

    Best regards from Dresden/Germany
    Viele Grüße aus Dresden
    Heiko Schlittermann
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