I installed a VM with Debian testing 64-bit. I have build-essential and crossbuild-essential-i386 installed.
I'm following the instructions here with one minor difference:
I installed lib32stdc++-8-dev instead of lib32stdc++-7-dev as I couldn't install 7 on this version of Debian.
Oddly enough I just setup a build environment on Windows 10 and was able to build sourcemod there with no issues there. But here is the compile failure I get on Linux: