Surely.
I have added
#ifndef QT_STATICPLUGIN
#define QT_STATICPLUGIN
#endif
to the header, but nothing changed. Still could not load.
Of course, qmake knows some magic options for g++ command. The question is, what commands?

Right now, I removed ^M from mb.cpp and use these two commands to create mb.so.

g++ -Wall -fPIC -c mb.cpp
g++ -Wall -fPIC -shared mb.o -o mb.so

But nothing changed:
Could not load ///.
What else can be suggested?

"...The compilation of loadable modules are very much system-dependent..."
http://www.inf.usi.ch/carzaniga/sdl/doc/index.html
Can anybody help with it?

I hope I don't need to add -I///directory flags to g++, because if they were required, compilation wouldn't occur (header files wouldn't be found). So, what do optimisation -O3, -pipe, -fno-omit-frame-pointer and -fno-optimize-sibling-calls flags mean? Could they fix the problem?