cataract: add -fcommon workaround

Without the change -fno-common toolchains like upstream gcc-10
fail to build as:

    ld: stats.o:/build/cataract-675e647/src/stats.h:24: multiple definition of
      `stats_images'; cgg.o:/build/cataract-675e647/src/stats.h:24: first defined here

pkgs/applications/misc/cataract/build.nix

@@ -26,6 +26,12 @@ stdenv.mkDerivation {
     sed -i 's|#include <exiv2/exif.hpp>|#include <exiv2/exiv2.hpp>|' src/jpeg-utils.cpp
   '';
 
+  # Add workaround for -fno-common toolchains like upstream gcc-10 to
+  # avoid build failures like:
+  #   ld: stats.o:/build/cataract-675e647/src/stats.h:24: multiple definition of
+  #     `stats_images'; cgg.o:/build/cataract-675e647/src/stats.h:24: first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   installPhase = ''
     mkdir $out/{bin,share} -p
     cp src/cgg{,-dirgen} $out/bin/