Accounting app won't build due to Qwt

[Robin R. <rob...@ci...>]

On Win64 am trying to build an old open source accounting package I 
found on the Internet called QHacc4. It uses Qwt, which I am able to build.

After some fixes, everything in app builds now except:

mingw32-make[1]: Entering directory 
'build-QHacc4-Desktop_Qt_5_13_1_MinGW_64_bit-Debug/gui'
C:/Qt/Tools/mingw730_64/bin/mingw32-make -f Makefile.Debug
mingw32-make[2]: Entering directory 
'build-QHacc4-Desktop_Qt_5_13_1_MinGW_64_bit-Debug/gui'
mingw32-make[2]: Leaving directory 
'build-QHacc4-Desktop_Qt_5_13_1_MinGW_64_bit-Debug/gui'
mingw32-make[2]: *** No rule to make target 'qwt/qwt_plot.h', needed by 
'build/debug/uis/ui_PlotWindow.h'.  Stop.
mingw32-make[1]: *** [Makefile:38: debug] Error 2

Clicking on the error in the IDE doesn't go anywhere. I see 
qwt/qwt_plot.h in some of the app *.ui files.

An installation/.pro issue? I added this to my .pro file:

     include ( qwt/qwt.prf )
     INCLUDEPATH += $$PWD

Am new to Qwt. How to figure out what to do next? How do we fix this?

Robin