The additional plugin style  code as cloned from https://code.qt.io/cgit/qt/qtstyleplugins.git/ fails to compile against Qt 5.15.2. The same plugin code compiles with Qt 5.14.2 without problems. Here is the part of the build log containing error:

qgtkstyle.cpp: In member function 'virtual void QGtkStyle::polish(QApplication*)':
qgtkstyle.cpp:443:30: error: 'setSystemPalette' is not a member of 'QApplicationPrivate'
443 | QApplicationPrivate::setSystemPalette(standardPalette());

Basically QApplicationPrivate::setSystemPalette which seems to be removed somewhere between 5.14.2 and 5.15.2 is referred at least two locations in the mentioned style plugin code.