networkmanager_strongswan: Format the expression

And use placeholders instead of Makefile variable interpolation.
Also move environment variable near each other.

pkgs/tools/networking/networkmanager/strongswan/default.nix

@@ -1,5 +1,15 @@
-{ lib, stdenv, fetchurl, intltool, pkg-config, networkmanager, strongswanNM
-, gtk3, gnome, libsecret, libnma }:
+{ stdenv
+, lib
+, fetchurl
+, intltool
+, pkg-config
+, networkmanager
+, strongswanNM
+, gtk3
+, gnome
+, libsecret
+, libnma
+}:
 
 stdenv.mkDerivation rec {
   pname = "NetworkManager-strongswan";
@@ -10,26 +20,35 @@ stdenv.mkDerivation rec {
     sha256 = "0sc1yzlxjfvl58hjjw99bchqc4061i3apw254z61v22k4sajnif8";
   };
 
-  buildInputs = [ networkmanager strongswanNM libsecret gtk3 libnma ];
-
-  nativeBuildInputs = [ intltool pkg-config ];
+  nativeBuildInputs = [
+    intltool
+    pkg-config
+  ];
 
-  # glib-2.62 deprecations
-  NIX_CFLAGS_COMPILE = "-DGLIB_DISABLE_DEPRECATION_WARNINGS";
+  buildInputs = [
+    networkmanager
+    strongswanNM
+    libsecret
+    gtk3
+    libnma
+  ];
 
   configureFlags = [
     "--without-libnm-glib"
     "--with-charon=${strongswanNM}/libexec/ipsec/charon-nm"
-    "--with-nm-libexecdir=$(out)/libexec"
-    "--with-nm-plugindir=$(out)/lib/NetworkManager"
+    "--with-nm-libexecdir=${placeholder "out"}/libexec"
+    "--with-nm-plugindir=${placeholder "out"}/lib/NetworkManager"
   ];
 
+  PKG_CONFIG_LIBNM_VPNSERVICEDIR = "${placeholder "out"}/lib/NetworkManager/VPN";
+
+  # glib-2.62 deprecations
+  NIX_CFLAGS_COMPILE = "-DGLIB_DISABLE_DEPRECATION_WARNINGS";
+
   passthru = {
     networkManagerPlugin = "VPN/nm-strongswan-service.name";
   };
 
-  PKG_CONFIG_LIBNM_VPNSERVICEDIR = "$(out)/lib/NetworkManager/VPN";
-
   meta = with lib; {
     description = "NetworkManager's strongswan plugin";
     inherit (networkmanager.meta) platforms;