dbus: enable building with musl

When building dbus with musl, it must not take systemd as a buildInput,
because systemd is not able to be build with musl.

pkgs/development/libraries/dbus/default.nix

@@ -3,6 +3,7 @@
 , fetchurl
 , pkgconfig
 , expat
+, enableSystemd ? stdenv.isLinux && !stdenv.hostPlatform.isMusl
 , systemd
 , libX11 ? null
 , libICE ? null
@@ -15,6 +16,8 @@ assert
   x11Support ->
     libX11 != null && libICE != null && libSM != null;
 
+assert enableSystemd -> systemd != null;
+
 stdenv.mkDerivation rec {
   pname = "dbus";
   version = "1.12.16";
@@ -50,11 +53,12 @@ stdenv.mkDerivation rec {
     expat
   ];
 
-  buildInputs = lib.optionals x11Support [
-    libX11
-    libICE
-    libSM
-  ] ++ lib.optional stdenv.isLinux systemd;
+  buildInputs =
+    lib.optionals x11Support [
+      libX11
+      libICE
+      libSM
+    ] ++ lib.optional enableSystemd systemd;
   # ToDo: optional selinux?
 
   configureFlags = [