Compiling nullmailer in OpenBSD

I thought I'd try nullmailer on a small PC running OpenBSD 6.0 -stable
but it stops at this point:

*======================================================================*
 test1:/usr/local/src/nullmailer-2.0
 {43} % make
 make  all-recursive
 Making all in doc
 Making all in lib
 Making all in cli++
 Making all in fdbuf
 Making all in mystring
 g++ -DHAVE_CONFIG_H -I. -I..      -g -O2 -fno-rtti -fno-exceptions -W -Wall -MT forkexec.o -MD -MP -MF .deps/forkexec.Tpo -c -o forkexec.o forkexec.cc
 In file included from forkexec.cc:33:
 forkexec.h: In member function 'void fork_exec::kill(int)':
 forkexec.h:32: error: '::kill' has not been declared
 *** Error 1 in lib (Makefile:429 'forkexec.o')
 *** Error 1 in lib (Makefile:474 'all-recursive')
 *** Error 1 in . (Makefile:389 'all-recursive')
 *** Error 1 in /usr/local/src/nullmailer-2.0 (Makefile:305 'all')
 test1:/usr/local/src/nullmailer-2.0
 {44} % 
*======================================================================*

I'm hoping this is a trivial problem, since it probably doesn't deserve
spending much time on it.

Thanks in advance!

Emilio