FreeBSD 10, exim complie error: perl.c:(.text+0x17c0): undefined reference to `Perl_save_strlen'

When compiling exim on FreeBSD 10, if you get this error:

gcc -o exim
perl.o: In function `call_perl_cat':
perl.c:(.text+0x17c0): undefined reference to `Perl_save_strlen'
collect2: error: ld returned 1 exit status
*** Error code 1

it's due to exim 4.85 not yet supporting perl 5.20.

Reported fix:

pkg install perl5.18
cd /usr/local/directadmin/custombuild
mkdir -p custom/exim
wget -O custom/exim/Makefile http://files1.directadmin.com/services/custombuild/Makefile

Then the following line needs to be added to custom/exim/Makefile:
PERL_COMMAND=/usr/local/bin/perl5.18.4

and after that, the ./build exim should work.