Require CMake 3.20+, because after upgrading the Android Gradle plugin
build with CMake <3.20 fails with the following error:
Execution failed for task ':app:buildCMakeDebug'.
> Expected output file at (...)obj/armeabi-v7a/libSDL2.so for target
SDL2 but there was none
Playlist is written to <gamedir>/playlist.txt during installation
("cdrom_device" in .xsys35rc is not used).
Files that match /(\d+)\.(wav|mp3|ogg)$/ are recognized as BGM files
(digits represent a track number). Note that this convention is the same
as Kichikuou on Web.