build,qt: Allow building QGpgME for Qt 6
* configure.ac: Look for Qt 5 and/or Qt 6. Require C++17 if Qt 6 binding is built. Build cmake files QGpgmeConfig* for Qt 5 and QGpgmeQt6Config* for Qt 6. (available_languages): Add "qt5" and "qt6". (WANT_QT5, WANT_QT6): New conditionals. * lang/qt/src/Makefile.am: Keep building libqgpgme for Qt 5. Build libqgpgmeqt6 for Qt 6. * lang/qt/tests/Makefile.am: Build tests for Qt 5 or Qt 6. * lang/qt/src/QGpgmeQt6Config-w32.cmake.in.in, lang/qt/src/QGpgmeQt6Config.cmake.in.in, lang/qt/src/QGpgmeQt6ConfigVersion.cmake.in, m4/qt6.m4: New.
This makes it possible to build QGpgME optionally for Qt 6.4.0 or later.
By default or if the language "qt" is enabled, then QGpgME is built
either for Qt 5 (if found) or Qt 6. A build for Qt 5 or Qt 6 can be
requested by explicitly enabling the language "qt5" or "qt6". Building
QGpgME for Qt 5 and Qt 6 simultaneously is not supported.