lsh: add -fcommon workaround

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

    ld: liblsh.a(unix_user.o):/build/lsh-2.0.4/src/server_userauth.h:108: multiple definition of
      `server_userauth_none_preauth'; lshd.o:/build/lsh-2.0.4/src/server_userauth.h:108: first defined here

pkgs/tools/networking/lsh/default.nix

@@ -31,7 +31,12 @@ stdenv.mkDerivation rec {
     export lsh_cv_sys_unix98_ptys=yes
   '';
 
-  NIX_CFLAGS_COMPILE = "-std=gnu90";
+  # -fcommon: workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: liblsh.a(unix_user.o):/build/lsh-2.0.4/src/server_userauth.h:108: multiple definition of
+  #     `server_userauth_none_preauth'; lshd.o:/build/lsh-2.0.4/src/server_userauth.h:108: first defined here
+  # Should be present in upcoming 2.1 release.
+  NIX_CFLAGS_COMPILE = "-std=gnu90 -fcommon";
 
   buildInputs = [ gperf guile gmp zlib liboop readline gnum4 pam ];