First you have to select compiler family (vcc, gcc, clang, etc.). Then provide executable paths. Then the options. E.g.
Emscripten (clang family):
nim c --cc:clang --clang.exe=emcc --clang.linkerexe=emcc myFile.nim
MXE cross-compiler (gcc family):
nim c --cc:gcc --gcc.exe=i686-w64-mingw32.static-gcc
--gcc.linkerexe=i686-w64-mingw32.static-gcc myFile.nim
Etc.
