Without the change the build fails on -fno-common toolchains like gcc-10 as:
$ nix build --impure --expr 'with import ./. {}; g15daemon.override { stdenv = clang13Stdenv; }'
ld: g15daemon.h:218: multiple definition of `lcdlist_mutex'; utility_funcs.o:g15daemon.h:218: first defined here
ld: g15daemon.h:203: multiple definition of `lcdnode_s'; utility_funcs.o:g15daemon.h:203: first defined here
