Just for modern perspective... you can use SDCC. It's an open-source optimizing C compiler targeting small microprocessors like the Z80 and various others. The project itself is terribly run--the maintainers recently pushed out an ABI change which broke everyone's code, but released it as a minor version bump. This ABI change did speed up the code, but that's small consolation to anyone who ended up with broken code.

IMO the Z80 is a lot nicer compiler target than the 6502, because the stack pointer is 16-bit and it's much easier to use the stack in general.

There are a couple C compilers for 6502 (like cc65 and WDC's C compiler) but they're not quite as good as SDCC, as far as I can tell. They're also not as actively maintained.