Fix gcc15 stricter check for headers

[mikelolasagasti]

This patch has been required since Fedora-42 adopted GCC15.

[thegushi]

Heyo, what platform did you test this on? I'm not sure how it would play on some of the other "special" platforms.

[mikelolasagasti]

Patch was required for Fedora 42, but it's been applied also to F41, F40, EPEL10 & EPEL9.

The best solution would be to rely on pkgconfig if libspf2 would provide it, but it's not the case.

[futatuki]

With https://github.com/shevek/libspf2/tree/d14abff4b544cfc53a8b5ef54cbc2353866b5081 (master at 2025-05-06), doing ./configure --prefix=/foo and then doing make install then spf.h will be installed as /foo/include/spf2/spf.h.

So the difference between without this change and with it, is only difference of the argment of --with-spf2-include we should pass on doing configure of OpenDMARC. For example, in the example above we shoud pass --with-spf2-include=/foo/include --with-spf2-lib=/foo/lib if the patch applied otherwise --with-spf2-include=/foo/include/spf2 --with-spf2-lib=/foo/lib.

I don't think this is related to gcc15.

[futatuki]

So the difference between without this change and with it, is only difference of the argment of --with-spf2-include we should pass on doing configure of OpenDMARC. For example, in the example above we shoud pass --with-spf2-include=/foo/include --with-spf2-lib=/foo/lib if the patch applied otherwise --with-spf2-include=/foo/include/spf2 --with-spf2-lib=/foo/lib.

My bad, it is incorrect. It affects the result of the configure: in the formar case, it adds '-I /foo/include' to CFLAGS in Makefile, and in the later case, it adds '-I /foo/include/spf2'. More over with this patch, with './configure --with-spf -with-spf2-include=/foo/include --with-spf2-lib=/foo/lib' brings build error:

libtool: compile:  cc -DHAVE_CONFIG_H -I. -I.. -I. -I.. -DCONFIG_BASE=\"/usr/local/etc\" -I /foo/include -g -O2 -MT opendmarc_dns.lo -MD -MP -MF .deps/opendmarc_dns.Tpo -c opendmarc_dns.c  -fPIC -DPIC -o .libs/opendmarc_dns.o
In file included from opendmarc_dns.c:16:
./opendmarc_internal.h:251:10: fatal error: 'spf.h' file not found
  251 | #include "spf.h"
      |          ^~~~~~~
1 error generated.
*** Error code 1

(I'm using FreeBSD 14.2 for confirmation, however, I believe this is generic problem on C compiler driver which support -I option)