Without the change build fails on -fno-common toolchains like upstream
gcc-10:
$ nix build -L --impure --expr 'with import ./. {}; gtkdialog.overrideAttrs (oa: { NIX_CFLAGS_COMPILE = (oa.NIX_CFLAGS_COMPILE or []) ++ ["-fno-common"]; })'
...
ld: widget_window.o:/build/gtkdialog-0.8.3/src/gtkdialog.h:43:
multiple definition of `function_signals_block'; gtkdialog.o:/build/gtkdialog-0.8.3/src/gtkdialog.h:43: first defined here