Since you seem to be using Linux, it would be better to leave the package with your distribution. Getting packages installed bringing static libraries into the system would be more "vanilla".
However, if you wish to stick to the in-project build principle, I would suggest not to disable the shadow build and create other workarounds having their own limitations.
You could for instance use the $$OUT_PWD variable for this purpose, but in general, you would need to specify the target dependency as well between the components.
So, you would be writing something like this then:
LIBS += -L$$OUT_PWD/qscintilla/Qt4Qt5 -lqscintilla2
Please adjust this based on your directory layout.