stellarium: make qtwebengine an optional dependency

This commit makes `qtwebengine` an optional dependency for stellarium. Since
`qtwebengine` currently fails to build on Darwin, it is now disabled by default
for that platform.

pkgs/applications/science/astronomy/stellarium/default.nix

@@ -22,6 +22,7 @@
 , nlopt
 , testers
 , xvfb-run
+, withQtWebEngine ? !stdenv.hostPlatform.isDarwin
 }:
 
 stdenv.mkDerivation (finalAttrs: {
@@ -65,7 +66,6 @@ stdenv.mkDerivation (finalAttrs: {
     qtpositioning
     qtmultimedia
     qtserialport
-    qtwebengine
     calcmysky
     qxlsx
     indilib
@@ -74,6 +74,8 @@ stdenv.mkDerivation (finalAttrs: {
     nlopt
   ] ++ lib.optionals stdenv.hostPlatform.isLinux [
     qtwayland
+  ] ++ lib.optionals withQtWebEngine [
+    qtwebengine
   ];
 
   preConfigure = ''