There is a cache directory on Windows, but I'm not sure what it actually maps to... It's FOLDERID_InternetCache/SyncTERM/<bbs name=""></bbs> The BBS name comes from the SyncTERM list entry (what you see when you open the program).
It seems like it's having problems finding the SDL framework. It looks in /Library/Frameworks/SDL2.framework by default, but you can set the directory SDL2.framework is in by adding SDL_FRAMEWORK_PATH=/path/to/dir on the command-line (do not include SDL2.framework in the path, default is /Library/Frameworks).
gmake[1]: unzip: No such file or directory You need to install the unzip package.
You can currently put them in: ~/.syncterm/cache/<BBS Name>, but that location may change in the future.
Could you rebuild with VERBOSE=1 passed on the make command-line and provide the ld command used? The manpage for pthread_yield on Gentoo may help as well. pthread_yield() is a terrible non-portable function, but it's always been available on Linux as far as I've seen.
I believe SkyPix is enabled with the RIP setting.
http://syncterm.net/ either build it yourself or download a nightly.
In C64 mode, you need to use a C64 ASCII 1963 font. If you use an ASCII 1968 font such as CP437, you'll find that characters are incorrect.