using clang, I still get the json error, if I skip that check, I’ll end up at a qt compilation error (clang++.exe: error: unknown argument: '-fno-keep-inline-dllexport') but if I run configure with the -nogui option, the build runs through (with some warnings, as you mentioned)
Another strange phenomenon I noticed after the compilation, If I try to run one of the compiled binaries (e.g. mrinfo), I get a blank output from the shell. If I run it from outside the shell (e.g. windows cmd), I get an error entry point XXX could not be located in the dynamic link library libstdc++-6.dll but this might be caused by the path-envorinment. Inside the mingw-shell I get /mingw64/bin/libstdc++-6.dll as path to the library which seems normal to me I guess.