Bug#915122: extra-cmake-modules: FindQHelpGenerator does not work for cross compilation

[Helmut Grohne <helmut@subdivi.de>]

Package: extra-cmake-modules
Version: 5.51.0-1
File: /usr/share/ECM/find-modules/FindQHelpGenerator.cmake
Control: affects -1 + src:kconfig
User: helmutg@debian.org
Usertags: rebootstrap

kconfig fails to cross build from source, because it fails to find
qhelpgenerator using extra-cmake-modules. The failure is due to a
combination of assumptions that don't add up. It's not entirely clear
which of them is wrong so I'm filing the bug with extra-cmake-modules
now even though it might need a fix elsewhere.

kconfig uses FindQHelpGenerator. FindQHelpGenerator looks up Qt5::qmake.
Since fixing #913499, qtbase-5-dev supplies
/usr/lib/$DEB_HOST_MULTIARCH/qt5/bin/qmake, which is a symbolic link to
/usr/bin/$DEB_HOST_GNU_TYPE-qmake, which is our cross wrapper for qmake.
Now FindQHelpGenerator.cmake uses the directory part of Qt5::qmake and
expects to find a qhelpgenerator inside that directory. Unfortunately,
it doesn't find one. qhelpgenerator is only located in /usr/bin,
/usr/lib/qt5/bin and /usr/lib/$DEB_BUILD_GNU_TYPE/qt5/bin.

The assumption of FindQHelpGenerator.cmake that the directory of
Qt5::qmake contains qhelpgenerator is not presently a valid one. I
cannot tell whether it should be valid or not.

If it should be valid, then some qt package should provide an additional
symlink for qhelpgenerator. Otherwise FindQHelpGenerator should use a
different way for finding qhelpgenerator.

I've Cced Dmitry and Lisandro and hope that they'll weigh in on how this
is supposed to work. Then we can figure out where this needs to be fixed
and how.

Thanks for your help

Helmut