Fix the generated qxmpp pkgconfig files

[krop]

The include directory and link targets changed. Also add a backward compatible pkgconfig file for Qt5 builds.

PR check list:

• [ x] Document your code
• [ x] Add \since QXmpp 1.X, QXMPP_EXPORT
• [ x] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [ x] Update doc/doap.xml
• [ x] Add unit tests
• [ x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[lnjX]

Thanks for fixing! Can you rebase upon the 1.5 branch?

[krop]

replaced by https://github.com/qxmpp-project/qxmpp/pull/568