BUG #15964: vacuumdb.c:187:10: error: use of undeclared identifier 'FD_SETSIZE'

The following bug has been logged on the website:

Bug reference: 15964
Logged by: sean
Email address: jungleboogie0(at)gmail(dot)com
PostgreSQL version: 12beta3
Operating system: kern.version=OpenBSD 6.6-beta (GENERIC.MP) #221: F
Description:

Hi All,

I'm trying to compile Postgresql from master on my OpenBSD machine, but I'm
getting a build failure here:
vacuumdb.c:184:26: error: use of undeclared identifier 'FD_SETSIZE'
if (concurrentCons > FD_SETSIZE - 1)
^
vacuumdb.c:187:10: error: use of undeclared identifier 'FD_SETSIZE'
FD_SETSIZE
- 1);
^
2 errors generated.
gmake[3]: *** [<builtin>: vacuumdb.o] Error 1
gmake[3]: Leaving directory '/home/sean/bin/postgresql/src/bin/scripts'
gmake[2]: *** [Makefile:41: all-scripts-recurse] Error 2
gmake[2]: Leaving directory '/home/sean/bin/postgresql/src/bin'
gmake[1]: *** [Makefile:42: all-bin-recurse] Error 2
gmake[1]: Leaving directory '/home/sean/bin/postgresql/src'
gmake: *** [GNUmakefile:11: all-src-recurse] Error 2
sean(at)beginning:postgresql$ sysctk kern.version

OS information:
$ sysctl kern.version
kern.version=OpenBSD 6.6-beta (GENERIC.MP) #221: Fri Aug 16 16:00:01 MDT
2019
deraadt(at)amd64(dot)openbsd(dot)org:/usr/src/sys/arch/amd64/compile/GENERIC.MP

gmake information:
$ gmake -v
GNU Make 4.2.1
Built for x86_64-unknown-openbsd6.6
Copyright (C) 1988-2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

clang information:
$ cc -v
OpenBSD clang version 8.0.0 (tags/RELEASE_800/final) (based on LLVM 8.0.0)
Target: amd64-unknown-openbsd6.6
Thread model: posix
InstalledDir: /usr/bin

Posgresql commit: d78d452bc5ac46a970e4fca2b31f3d533815c39a

config options:
./configure --enable-cassert --enable-debug --with-perl \
--with-python --with-tcl --with-tclconfig=/usr/local/lib/tcl/tcl8.6/
\
--with-includes=/usr/local/include

What am I doing wrong? What more information would you like?

https://www.postgresql.org/docs/current/bug-reporting.html
5.1 states:
PostgreSQL fails to compile, build, or install according to the instructions
on supported platforms.

Thanks!

P.S. The submit bug page does not list master/current/trunk as a version,
and I needed to make a selection, which doesn't match the commit I
referenced above. Fear not, I am using the git repo here:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=summary and cloned it
locally.