ppp: add withSystemd option

Not all platforms support systemd.  This fixes e.g. pkgsStatic.ppp.

Fixes: 8990531424 ("ppp: add systemd sd-notify support")

pkgs/by-name/pp/ppp/package.nix

@@ -10,6 +10,7 @@
   openssl,
   pkg-config,
   stdenv,
+  withSystemd ? lib.meta.availableOn stdenv.hostPlatform systemdMinimal,
   systemdMinimal,
 }:
 
@@ -28,7 +29,7 @@ stdenv.mkDerivation rec {
     "--localstatedir=/var"
     "--sysconfdir=/etc"
     "--with-openssl=${openssl.dev}"
-    "--enable-systemd"
+    (lib.enableFeature withSystemd "systemd")
   ];
 
   nativeBuildInputs = [
@@ -36,14 +37,17 @@ stdenv.mkDerivation rec {
     autoreconfHook
   ];
 
-  buildInputs = [
-    bash
-    libpcap
-    libxcrypt
-    linux-pam
-    openssl
-    systemdMinimal
-  ];
+  buildInputs =
+    [
+      bash
+      libpcap
+      libxcrypt
+      linux-pam
+      openssl
+    ]
+    ++ lib.optionals withSystemd [
+      systemdMinimal
+    ];
 
   postPatch = ''
     for file in $(find -name Makefile.linux); do