Thank you! Now I'm much more close to the solution. I use 3rd way from wiki, include to my .pro:

INCLUDEPATH += C:/qextserialport-1.2/src
LIBS += -L C:/qextserialport-1.2/lib -lqextsrialport-1.2
DEFINES += QEXTSERIALPORT_USING_SHARED
I've truncated my code and left just the open/close procedure by press Button. Now I have only unique error:

:-1: error: LNK1146: no argument specified with option '/LIBPATH:'

And I really do not see any lib path in the proper directory. But I used instructions and download the directory from one issue that you wrote (instructions for QExtSerialPort)