The binaries in this folder are built from mGBA, an emulator for running Game Boy Advance games. The source code is available here: https://github.com/mgba-emu/mgba. The source code for these specific builds is available from:

Windows: https://github.com/mgba-emu/mgba/tree/7ee2be6c96222dca12a9a579b747fe5ff1829def
Linux: https://github.com/mgba-emu/mgba/tree/dbffb46c4e7d2e7a2cbed7c3488cece4c2176d4c
Mac: https://github.com/mgba-emu/mgba/tree/daf01b03d5316dac966acd4b05318a225cab12f5