emulators/flexemu: Update to 3.30

• Change from autotools to cmake
• Change from Qt5 to Qt6
• Compiler needs C++17
• Preparations for test target (not buildable atm.)

Changelog: https://github.com/aladur/flexemu/commit/3b579e5014a2eb3a342b47c04047023cb9935a2b

Reported by: portscout, Repology