_9pfs: add -fcommon workaround

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

    ld: lib/auth_rpc.o:/build/source/lib/../9pfs.h:35: multiple definition of
        `logfile'; 9pfs.o:/build/source/9pfs.h:35: first defined here

pkgs/tools/filesystems/9pfs/default.nix

@@ -24,6 +24,12 @@ stdenv.mkDerivation {
 
   buildInputs = [ fuse ];
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: lib/auth_rpc.o:/build/source/lib/../9pfs.h:35: multiple definition of
+  #     `logfile'; 9pfs.o:/build/source/9pfs.h:35: first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   enableParallelBuilding = true;
 
   meta = {