Build KDE 6 on Ubuntu 22.04

To build KDE 6 on Ubuntu 22.04, other than steps on https://community.kde.org/Get_Involved/development/Build_software_with_kdesrc-build, there are other things need to be noticed:

• Qt6 in apt repo is not new enough, should let kdesrc-build build Qt6; follow instruction at https://community.kde.org/Get_Involved/development/More#Build_Qt6_using_kdesrc-build, and notice some packages are required to build some parts (see below)
• gpgme fail to build, check https://community.kde.org/Get_Involved/development/More#kdesrc-build_issues, or use environment variable to help pkg-config find Qt 6. Notice this should point to installed location, like

export PKG_CONFIG_PATH=/opt/kde/usr/lib/pkgconfig:$PKG_CONFIG_PATH

• KWin requires libdisplay-info, install it from newer repo, e.g. Debian Sid; or check the link above
• KWin requires Qt6 built with atspi_accessibility, which requires ATSPI2, so need to install libatspi2.0-dev before build Qt6
• plasma-desktop: SDL2 in Ubuntu 22.04 not new enough to have SDL_JoystickPath, disable game controller support in kcms/CMakeLists.txt
• xdg-desktop-portal-kde requires QtPrintSupport/private/qcups_p.h, so install libcups2-dev before building Qt6