qt5.qtwebkit: link against libicucore.dylib from darwin.ICU instead of /usr/lib

The impure library was introduced in

   775531c1e3 ('qt58: extend darwin compatibility')

cmake-based build can use a non-apple ICU if -DMACOS_USE_SYSTEM_ICU=OFF
is set.

pkgs/development/libraries/qt-5/5.9/qtwebkit.patch

@@ -1,16 +1,3 @@
-diff --git a/Source/WTF/WTF.pri b/Source/WTF/WTF.pri
-index 69e4cd1f3..3f729a75e 100644
---- a/Source/WTF/WTF.pri
-+++ b/Source/WTF/WTF.pri
-@@ -12,7 +12,7 @@ mac {
-     # Mac OS does ship libicu but not the associated header files.
-     # Therefore WebKit provides adequate header files.
-     INCLUDEPATH = $${ROOT_WEBKIT_DIR}/Source/WTF/icu $$INCLUDEPATH
--    LIBS += -licucore
-+    LIBS += /usr/lib/libicucore.dylib
- } else:!use?(wchar_unicode): {
-     win32 {
-         CONFIG(static, static|shared) {
 diff --git a/Source/WebCore/plugins/qt/PluginPackageQt.cpp b/Source/WebCore/plugins/qt/PluginPackageQt.cpp
 index a923d49aa..46772a4bb 100644
 --- a/Source/WebCore/plugins/qt/PluginPackageQt.cpp

pkgs/development/libraries/qt-5/modules/qtwebkit.nix

@@ -27,7 +27,7 @@ qtModule {
     ++ optional (stdenv.isDarwin && lib.versionAtLeast qtbase.version "5.9.0") qtmultimedia
     ++ optional (lib.versionAtLeast qtbase.version "5.11.0") qtwebchannel;
   buildInputs = [ fontconfig libwebp libxml2 libxslt sqlite glib gst_all_1.gstreamer gst_all_1.gst-plugins-base ]
-    ++ optionals (stdenv.isDarwin) (with darwin; with apple_sdk.frameworks; [ cf-private OpenGL ])
+    ++ optionals (stdenv.isDarwin) (with darwin; with apple_sdk.frameworks; [ cf-private ICU OpenGL ])
     ++ optionals (lib.versionAtLeast qtbase.version "5.11.0") [ hyphen ];
   nativeBuildInputs = [
     bison2 flex gdb gperf perl pkgconfig python2 ruby
@@ -35,10 +35,6 @@ qtModule {
 
   cmakeFlags = optionals (lib.versionAtLeast qtbase.version "5.11.0") [ "-DPORT=Qt" ];
 
-  __impureHostDeps = optionals (stdenv.isDarwin) [
-    "/usr/lib/libicucore.dylib"
-  ];
-
   # QtWebKit overrides qmake's default_pre and default_post features,
   # so its custom qmake files must be found first at the front of QMAKEPATH.
   preConfigure = ''