Without the change build fails on -fno-common toolchains like upstream
gcc-10 as:
$ nix build -L --impure --expr 'with import ./. {}; gqview.overrideAttrs (oa: { NIX_CFLAGS_COMPILE = (["-fno-common"] ++ [oa.NIX_CFLAGS_COMPILE or ""]); })'
...
ld: pan-view.o:/build/gqview-2.1.5/src/bar_exif.h:27:
multiple definition of `bar_exif_key_list'; bar_exif.o:/build/gqview-2.1.5/src/bar_exif.h:27: first defined here