Add support to build with system libqscintilla

[iakov]

Some distros (Debian, Ubuntu, AltLinux) has libqscintilla2-qt5-dev onboard, and the Homebrew has qscintilla2 too. Make a CONFIG+=use_system_qscintilla option for this build

[iakov]

2024.1 has this options, but CI build configuration for "native" distro build with system packages must be implemented.