cgminer: 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/applications/blockchains/cgminer/default.nix

@@ -39,6 +39,13 @@ stdenv.mkDerivation rec {
                      "--enable-keccak"
                      "--enable-bflsc"];
 
+  # 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
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   meta = with lib; {
     description = "CPU/GPU miner in c for bitcoin";
     homepage = "https://github.com/ckolivas/cgminer";