Do pass -isysroot optionally for gn build

With cmake 4.0 changes, we can no longer expect CMAKE_OSX_SYSROOT
to be set:
https://cmake.org/cmake/help/latest/release/4.0.html#other-changes

Therefore make the setting optional for gn, as it should not be
required on "modern macOS".

Pick-to: 6.9 6.8
Task-number: QTBUG-135621
Change-Id: I48ed72d6e920fec4c395eb04c8a18619e045c1b4
Reviewed-by: Moss Heim <[email protected]>

Author: Michal Klocek

src/gn/CMakeLists.txt

@@ -79,8 +79,8 @@ add_custom_command(
         --platform ${platform}
         --ar ${CMAKE_AR}
         --qt-version "${QT_REPO_MODULE_VERSION}.qtwebengine.qt.io"
-        $<$<PLATFORM_ID:Darwin>:--isysroot>
-        $<$<PLATFORM_ID:Darwin>:${CMAKE_OSX_SYSROOT}>
+        $<$<AND:$<PLATFORM_ID:Darwin>,$<BOOL:${CMAKE_OSX_SYSROOT}>>:--isysroot>
+        $<$<AND:$<PLATFORM_ID:Darwin>,$<BOOL:${CMAKE_OSX_SYSROOT}>>:${CMAKE_OSX_SYSROOT}>
         $<$<AND:$<PLATFORM_ID:Darwin>,$<BOOL:${OSX_ARCH}>>:--osx-architectures>
         $<$<AND:$<PLATFORM_ID:Darwin>,$<BOOL:${OSX_ARCH}>>:${OSX_ARCH}>
         ${GN_GCC_LEGACY_SUPPORT}