adtool: add -fcommon workaround

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

    ld: ../../src/lib/libactive_directory.a(active_directory.o):/build/adtool-1.3.3/src/lib/active_directory.h:31:
      multiple definition of `system_config_file'; adtool.o:/build/adtool-1.3.3/src/tools/../../src/lib/active_directory.h:31: first defined here

pkgs/tools/admin/adtool/default.nix

@@ -19,6 +19,12 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ openldap ];
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: ../../src/lib/libactive_directory.a(active_directory.o):/build/adtool-1.3.3/src/lib/active_directory.h:31:
+  #     multiple definition of `system_config_file'; adtool.o:/build/adtool-1.3.3/src/tools/../../src/lib/active_directory.h:31: first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   enableParallelBuilding = true;
 
   postInstall = ''