cuneiform: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:

    ld: cgminer-driver-modminer.o:/build/source/miner.h:285:
      multiple definition of `bitforce_drv'; cgminer-cgminer.o:/build/source/miner.h:285:
      first defined here

pkgs/tools/graphics/cuneiform/default.nix

@@ -20,6 +20,12 @@ stdenv.mkDerivation {
   })
   ];
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: CMakeFiles/rbal.dir/src/statsearchbl.cpp.o:(.bss+0x0):
+  #     multiple definition of `minrow'; CMakeFiles/rbal.dir/src/linban.c.o:(.bss+0xa3a): first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   postPatch = ''
     rm cuneiform_src/Kern/hhh/tigerh/h/strings.h
   '';