The error message when the miniupnpc.h is not found is fubared due the
CMAKE_REQUIRED_INCLUDES not taking into account the NOTFOUND case. This
diff fixes the issue and restore the expected error.

Uninstall miniupnpc, then

mkdir buildcmake && cd buildcmake
cmake -GNinja ..

Should output a CMake error:

Could NOT find MiniUPnPc (missing: MINIUPNPC_INCLUDE_DIR
MINIUPNPC_LIBRARY)