I am one of the developers for PLPlot and I have been trying different build configurations to check where it does and does not work.
When I tried building on Windows 10 using MSYS2, I get the “System is unknown to cmake” message. The cmake identifies the system as "MINGW64_NT-10.0"
I managed to get cmake to generate a working Makefile with the following command line
cmake -G "Unix Makefiles" -DCMAKE_INSTALL_PREFIX=/opt/plplot -DPLD_wingdi=ON -DPLD_wingcc=ON -DWIN32=1 -DMINGW=1 -DMINGWLIBPATH=/d/msys64/usr/lib/w32api
../plplot/ &> cmake.log
I have attached the cache file per the instructions in the message. Please let me know if you need me to try anything.