Error on build for qwt 6.1.6

Hello,
I have been attempting install of QWT with version 6.1.6

Issued qmake with no error on directory.
On make I get this in the display at beginning of build;

$ make
cd src/ && ( test -e Makefile || /usr/local/Qt/6.0.1/gcc_64/bin/qmake -o Makefile /home/david/Downloads/qwt-6.1.6/src/src.pro ) && make -f Makefile
make[1]: Entering directory '/home/david/Downloads/qwt-6.1.6/src'
compiling qwt_clipper.cpp
qwt_clipper.cpp: In member function ‘QVector<QwtInterval> QwtCircleClipper::clipCircle(const QPointF&, double) const’:
qwt_clipper.cpp:354:9: error: ‘qSort’ was not declared in this scope; did you mean ‘qSqrt’?
354 | qSort( angles );
| ^~~~~
| qSqrt
make[1]: *** [Makefile:4637: obj/qwt_clipper.o] Error 1
make[1]: Leaving directory '/home/david/Downloads/qwt-6.1.6/src'
make: *** [Makefile:50: sub-src-make_first-ordered] Error 2

I have searched forums for same error and have come up with nothing.
I followed the installation instructions like so;

/usr/local/Qt/6.0.1/gcc_64/bin/qmake qwt.pro

then

make

Any pointers or suggestions would be welcome.