Thanks, Rich. That helps somewhat. Now the error is different:
$ export QMAKESPEC=/usr/local/lib/qt5/mkspecs
$ ./configure --prefix=/usr/local
checking for QT5 qmake... /usr/local/bin/qmake
checking for lrelease... /usr/local/bin/lrelease
checking for c++... /usr/local/bin/c++
checking whether c++ compiler builds test program... ok
checking whether c++ compiler supports -std=c++0x... ok
checking whether compiled test program works... ok
checking for make... /usr/local/bin/make
checking for pkg-config... /usr/local/bin/pkg-config
checking for Qt5Core library... -L/usr/local/lib -lQt5Core
checking for Qt5Gui library... -L/usr/local/lib -lQt5Gui -lQt5Core
checking for Qt5Network library... -L/usr/local/lib -lQt5Network -lQt5Core
checking for Qt5WebEngine library... not found!
checking for Qt5Widgets library... -L/usr/local/lib -lQt5Widgets -lQt5Gui -lQt5Core
checking for Qt5WebEngineWidgets library... not found!
checking for Qt5PrintSupport library... -L/usr/local/lib -lQt5PrintSupport -lQt5Widgets -lQt5Gui -lQt5Core
checking for Qt5Svg library... -L/usr/local/lib -lQt5Svg -lQt5Widgets -lQt5Gui -lQt5Core
checking for Qt5WebSockets library... -L/usr/local/lib -lQt5WebSockets -lQt5Network -lQt5Core
checking for Qt5WebChannel library... -L/usr/local/lib -lQt5WebChannel -lQt5Qml -lQt5Network -lQt5Core
checking for uchardet library... -L/usr/local/lib -luchardet
generate Makefile... Cannot read /usr/local/lib/qt5/common/linux.conf: No such file or directory
Cannot read /usr/local/lib/qt5/common/gcc-base-unix.conf: No such file or directory
Cannot read /usr/local/lib/qt5/common/g++-unix.conf: No such file or directory
Project ERROR: qmake spec does not announce the compiler family, and it cannot be guessed.
Error processing project file: notepadqq.pro
error!
I'm puzzled why qmake can't find things on its own.
Is it a problem with how qmake was packaged, or is it a problem with notepadqq's source code (making assumptions that are true for some distros but not true for TCL)?