Better workaround for warning with nix

[Luflosi]

When building kitty for nix, the compilation would fail:

building
CC: clang (4, 2)
[4/55] Compiling kitty/fonts.c ... done ...
Compiling kitty/screen.c ...
clang -MMD -DNDEBUG -DPRIMARY_VERSION=4000 -DSECONDARY_VERSION=16 -Wextra -Wno-missing-field-initializers -Wall -Wstrict-prototypes -std=c11 -pedantic-errors -Werror -O3 -fwrapv -fstack-protector-strong -pipe -march=native -fvisibility=hidden -D_FORTIFY_SOURCE=2 -pthread -I/nix/store/r66qbg89vfk21rilhx62ylviq982zf70-libpng-apng-1.6.37-dev/include/libpng16 -I/nix/store/wnp6bg1d6xf557200sc2hbkks2vkj3ll-zlib-1.2.11-dev/include -I/nix/store/5m21j8vqa45hr7dc99r8kxkqa7rpm4fh-harfbuzz-2.6.4-dev/include/harfbuzz -I/nix/store/9dswjy6wfc5qrmrpwp1pl986anc36bcn-python3-3.7.6/include/python3.7m -c kitty/screen.c -o build/fast_data_types-screen.c.o
clang-7: error: argument unused during compilation: '-fno-strict-overflow' [-Werror,-Wunused-command-line-argument]
builder for '/nix/store/r04zplcjdfc1rmswmrgg22mw1shh4bys-kitty.drv' failed with exit code 1
error: build of '/nix/store/r04zplcjdfc1rmswmrgg22mw1shh4bys-kitty.drv' failed

To work around this problem, the compiler command line options-pedantic-errors and -Werror are patched out of setup.py. I don't think that's a very good solution, so I wanted to change that. A slightly better workaround would be to add -Wno-unused-command-line-argument, which suppresses the warning but ideally we would just not pass the option in the first place. But I am puzzled why the warning is there. It warns, that the option -fno-strict-overflow was not used. But it is not even passed to clang and also doesn't occur anywhere in kitty's source code.
I also don't really know why this doesn't happen when I compile kitty outside of nix, maybe there is a difference between clang from Xcode and clang from nix.

[kovidgoyal]

This will be because nix's build system is adding flags to compiler
invocations behind the back. Presumably the clang that is invoked on nix
is a wrapper that calls the actual clang with added flags. There's
nothing that can be done about that in kitty, since we have no way
to detect modified clang implementations.

If I am correct nix needs to stop messing with compiler flags. If they
feel a flag should be present they need to work with upstream to get it
added.

NixOS/nixpkgs#18995
NixOS/nixpkgs#39687

I am certainly not going to add flags to kitty's build system to
accommodate this buggy behavior of nix.

[Luflosi]

Thank you, your tip was very helpful. I agree, this is not a problem that should be fixed in kitty. I found a way to disable that particular compiler flag in the build environment.