So, wondering why the guard around swab in W:\usr\lib\gcc\i686-pc-os2-emx\9\include-fixed\unistd.h fails for you yet the same one in unistd.h works, I thought to download the source.
Rather my sourceforge cookie expired or they've changed their code as all I get is the cloudflare verifying you are human thingy now. Tried on my phone, got something like "downloading to this device is not allowed" in 3 different browsers. Fell back to using Thunderbird, which actually still works. This is a problem on OS/2 as all our browsers fail to get by cloudflare now. Previously I've been asked for help downloading agena due to cloudflare and due to having a cookie, I could.
Next problem, makefile.os2 says
https://sourceforge.net/projects/agena/files/Packages/mapmagena-1.1.2a.tar.gz/download is a prerequisite. Sourceforge says it doesn't exist. So using Thunderbird, I found mapmagena-2.0.4.tar.gz in your files area.
I'll try compiling soon.
BTW, Arca Noae does not support GCC and recommends using OpenWatcom, see
https://www.arcanoae.com/wiki/information-for-developers/ Most of GCC development is done by Paul now, previously Bitwise who helped Paul and tweaked 9.2.0 and for klibc, originally IBM paid for it to build Mozilla.