I tried building the #C64 emulator VICE from sources, and it wasn't too hard (the hardest part was remembering how to use Subversion, lol).

In spite of being an ancient codebase, it's still actively developed, with several new features and fixes for the upcoming 3.6 release:
https://sourceforge.net/p/vice-emu/code/HEAD/tree/trunk/vice/NEWS
#retrocomputing