qt5.qtwebengine: disable platform check using patches

The previous code using substituteInPlace was broken for both 5.9 and
5.11 (latter was broken after a minor release bump). Using patches
should make things fail loudly and prevent modules/qtwebengine.nix from
being littered with version checks.

The patch for 5.12 is only tested to apply.

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

@@ -67,18 +67,8 @@ qtModule {
         src/3rdparty/chromium/gpu/config/gpu_info_collector_linux.cc
     ''
     + optionalString stdenv.isDarwin (''
-      # Remove annoying xcode check
-      substituteInPlace mkspecs/features/platform.prf \
-        --replace "lessThan(QMAKE_XCODE_VERSION, 7.3)" false \
-        --replace "/usr/bin/xcodebuild" "xcodebuild"
-
-      substituteInPlace src/3rdparty/chromium/build/mac_toolchain.py \
-        --replace "/usr/bin/xcode-select" "xcode-select"
-
       substituteInPlace src/core/config/mac_osx.pri \
-        --replace /usr ${stdenv.cc} \
-        --replace "isEmpty(QMAKE_MAC_SDK_VERSION)" false
-
+        --replace /usr ${stdenv.cc}
     ''
     # TODO remove when new Apple SDK is in
     + (if lib.versionOlder qtCompatVersion "5.11" then ''