Hi,
This occurs throughout Qt in later versions with newer compilers.
The way to fix it is to edit $HOME/qt-everywhere-src-5.12.6/qtbase/mkspecs/common/gcc-base.conf
Change QMAKE_CFLAGS_ISYSTEM = -isystem
to QMAKE_CFLAGS_ISYSTEM = -I (capital i)
Also, insert the following lines in the qmake.conf of the device:
QMAKE_CXXFLAGS += -std=gnu++11
QMAKE_CFLAGS_ISYSTEM=-I
Jaf