Without the change build fails on toolchains like clang-13 which
switch to -fno-common by default:
$ nix build -L --impure --expr 'with import ./. {}; read-edid.override { stdenv = clang13Stdenv; }'
...
read-edid> ld: CMakeFiles/get-edid.dir/classic.c.o:(.bss+0x0):
multiple definition of `quiet'; CMakeFiles/get-edid.dir/get-edid.c.o:(.bss+0x0): first defined here
read-edid> ld: CMakeFiles/get-edid.dir/i2c.c.o:(.bss+0x0):
multiple definition of `quiet'; CMakeFiles/get-edid.dir/get-edid.c.o:(.bss+0x0): first defined here
Sergei Trofimovich