Got this error.
libspf2 and opendmarc persist in system.

root@beta:/usr/ports/mail/exim # portmaster mail/exim
...
===>   exim-4.91 depends on shared library: libopendmarc.so - found (/usr/local/lib/libopendmarc.so)
===>   exim-4.91 depends on shared library: libspf2.so - found (/usr/local/lib/libspf2.so)
...
# define LIBICONV_PLUG
         ^
<command line>:1:9: note: previous definition is here
#define LIBICONV_PLUG 1
        ^
1 warning generated.
cc dmarc.c
In file included from dmarc.c:13:
In file included from ./exim.h:36:
./os.h:37:10: warning: 'LIBICONV_PLUG' macro redefined [-Wmacro-redefined]
# define LIBICONV_PLUG
         ^
<command line>:1:9: note: previous definition is here
#define LIBICONV_PLUG 1
        ^
dmarc.c:16:4: error: SPF must also be enabled for DMARC
#  error SPF must also be enabled for DMARC
   ^
1 warning and 1 error generated.
*** Error code 1

Stop.
make[3]: stopped in /usr/ports/mail/exim/work/exim-4.91/build-FreeBSD-amd64
*** Error code 1

Stop.
make[2]: stopped in /usr/ports/mail/exim/work/exim-4.91
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/mail/exim
*** Error code 1

Stop.
make: stopped in /usr/ports/mail/exim

===>>> make build failed for mail/exim
===>>> Aborting update

root@beta:/usr/ports/mail/exim # make showconfig | egrep "SPF|DMARC"
     DMARC=on: Enable DMARC support
     SPF=on: Enable Sender Policy Framework checking
root@beta:/usr/ports/mail/exim # uname -v
FreeBSD 11.1-RELEASE-p9 #0: Tue Apr  3 16:59:16 UTC 2018     root@amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC