bristol: add -fcommon workaround

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

    ld: brightonCLI.o:/build/bristol-0.60.11/brighton/brightonCLI.c:139: multiple definition of
      `event'; brightonMixerMenu.o:/build/bristol-0.60.11/brighton/brightonMixerMenu.c:1182: first defined here

pkgs/applications/audio/bristol/default.nix

@@ -19,6 +19,12 @@ stdenv.mkDerivation  rec {
 
   configurePhase = "./configure --prefix=$out --enable-jack-default-audio --enable-jack-default-midi";
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #  ld: brightonCLI.o:/build/bristol-0.60.11/brighton/brightonCLI.c:139: multiple definition of
+  #    `event'; brightonMixerMenu.o:/build/bristol-0.60.11/brighton/brightonMixerMenu.c:1182: first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   preInstall = ''
     sed -e "s@\`which bristol\`@$out/bin/bristol@g" -i bin/startBristol
     sed -e "s@\`which brighton\`@$out/bin/brighton@g" -i bin/startBristol