Bug#918760: agree on how to find qhelpgenerator

[Helmut Grohne <helmut@subdivi.de>]

Package: qt5-qmake,qttools5-dev-tools,extra-cmake-modules
User: helmutg@debian.org
Usertags: rebootstrap
Control: affects -1 + src:kpackage

kpackage fails to cross build from source and the cause is tricky. We
start our journey on extra-cmake-modules'
/usr/share/ECM/find-modules/FindQHelpGenerator.cmake. That file assumes
that it will find a "qhelpgenerator" executable next to the qmake
executable. The path of qmake is determined using

    get_target_property(_qmake_EXECUTABLE Qt5::qmake LOCATION)

and results in:

    /usr/lib/<hosttriplet>/qt5/bin/qmake

Now, qttools5-dev-tools places qhelpgenerator in

    /usr/lib/qt5/bin/qhelpgenerator
    /usr/lib/<buildtriplet>/qt5/bin/qhelpgenerator

and qtchooser additionally makes it available as:

    /usr/bin/qhelpgenerator

The end result is that qhelpgenerator is not found and that kpackage
fails to cross build.

extra-cmake-modules's assumption on how to find qhelpgenerator is not
presently valid. We'll either have to reinstate the assumption or make
extra-cmake-modules not assume that.

Now I need your input on which package we need to touch.

Helmut