webkitgtk: honor systemdSupport

This commit exposes support for compilation without systemd,
controlled by the global systemdSupport argument.  This argument is
understood by many other nixpkgs expressions and can be set globally
in ~/.config/nixpkgs/config.nix.

pkgs/development/libraries/webkitgtk/default.nix

@@ -60,6 +60,7 @@
 , addOpenGLRunpath
 , enableGeoLocation ? true
 , withLibsecret ? true
+, systemdSupport ? stdenv.isLinux
 }:
 
 stdenv.mkDerivation rec {
@@ -158,9 +159,10 @@ stdenv.mkDerivation rec {
     bubblewrap
     libseccomp
     libmanette
-    systemd
     wayland
     xdg-dbus-proxy
+  ] ++ lib.optionals systemdSupport [
+    systemd
   ] ++ lib.optionals enableGeoLocation [
     geoclue2
   ] ++ lib.optionals withLibsecret [
@@ -193,7 +195,7 @@ stdenv.mkDerivation rec {
     "-DUSE_APPLE_ICU=OFF"
     "-DUSE_OPENGL_OR_ES=OFF"
     "-DUSE_SYSTEM_MALLOC=ON"
-  ] ++ lib.optionals (!stdenv.isLinux) [
+  ] ++ lib.optionals (!systemdSupport) [
     "-DUSE_SYSTEMD=OFF"
   ] ++ lib.optionals (stdenv.isLinux && enableGLES) [
     "-DENABLE_GLES2=ON"