libreswan: fix build with glibc 2.26

It is not trivial to update libreswan to 3.22 because it has added a dependency
on ldns which currently depends on openssl 1.1.0, while the rest of libreswan
depends on openssl 1.0.2.  It compiles, but may behave unpredictably at run time.

Tracking issue: #31696

pkgs/tools/networking/libreswan/default.nix

@@ -24,6 +24,9 @@ stdenv.mkDerivation {
     sha256 = "0zginnakxw7m79zrdvfdvliaiyg78zgqfqkks9z5d1rjj5w13xig";
   };
 
+  # These flags were added to compile v3.18. Try to lift them when updating.
+  NIX_CFLAGS_COMPILE = [ "-Wno-error=redundant-decls" "-Wno-error=format-nonliteral" ];
+
   nativeBuildInputs = [ makeWrapper pkgconfig ];
   buildInputs = [ bash iproute iptables systemd coreutils gnused gawk gmp unbound bison flex pam libevent
                   libcap_ng curl nspr nss python ]
@@ -42,11 +45,13 @@ stdenv.mkDerivation {
     # Fix python script to use the correct python
     sed -i -e 's|#!/usr/bin/python|#!/usr/bin/env python|' -e 's/^\(\W*\)installstartcheck()/\1sscmd = "ss"\n\0/' programs/verify/verify.in
   '';
-  
+
+  patches = [ ./libreswan-3.18-glibc-2.26.patch ];
+
   # Set appropriate paths for build
   preBuild = "export INC_USRLOCAL=\${out}";
 
-  makeFlags = [ 
+  makeFlags = [
     "INITSYSTEM=systemd"
     (if docs then "all" else "base")
   ];
@@ -64,7 +69,7 @@ stdenv.mkDerivation {
     done
   '';
 
-  enableParallelBuilding = false;
+  enableParallelBuilding = true;
 
   meta = {
     homepage = https://libreswan.org;