Fix build on macOS arm64

[rnkn]

Hello. There are a few things in this PR. The main changes are discussed in #64

The Makefile was overriding CFLAGS and LDFLAGS so I've changed those.

I've also changed the build process to not use OBJDIR. I found this a bit clunky:

printf "$(OBJDIR)/" > $@

and I think this is neater and permits resuming an interrupted build without running make clean and starting from scratch.

I've also required a system fmtlib instead of building in repo. I have not, however, removed the format.cc object required for building. (I don't really understand this tbh.)

I understand if you don't want to merge this. It's mainly for my own interest.

[rnkn]

I figured out how to build entirely with the system fmtlib, which makes the build even simpler because there are no CCFILES.