COMP: Fix macOS signing with webengine support disabled

This commit fix the following codesign error reported when attempting to
sign a Slicer application package build with Slicer_BUILD_WEBENGINE_SUPPORT
set to OFF.

For reference, the regression was introduced in 100f826 (BUG: Update
macOS fixup script to package QtWebEngineProcess.app) and exerted following
the updated of the macOS signing server to macOS 12.

Error:

  $ ./slicer-macos-codesign-scripts/sign.bash \
    ${identifier} ${version} \
  [...]
  --   Signing App
  /Volumes/tmp.9dkIOBNj/Slicer.app: bundle format unrecognized, invalid, or unsuitable
  In subcomponent: /Volumes/tmp.9dkIOBNj/Slicer.app/Contents/Frameworks/QtWebEngineCore.framework

CMake/SlicerCPackBundleFixup.cmake.in

@@ -319,10 +319,11 @@ function(fixup_bundle_with_plugins app)
     # Since BundleUtilities function do not copy the "Helpers" directory found
     # in frameworks. The following will explicitly copy and fixup
     # QtWebEngineCore.framework/Versions/5/Helpers/QtWebEngineProcess.app
+    set(Slicer_BUILD_WEBENGINE_SUPPORT "@Slicer_BUILD_WEBENGINE_SUPPORT@")
     set(qtwebenginecore_app_subdir "Helpers/QtWebEngineProcess.app")
     set(qtwebenginecore_src_dir "${qt_root_dir}/QtWebEngineCore.framework/${qtwebenginecore_app_subdir}")
     set(qtwebenginecore_dest_dir "${app_dir}/Contents/Frameworks/QtWebEngineCore.framework/Versions/Current/${qtwebenginecore_app_subdir}")
-    if(EXISTS ${qtwebenginecore_src_dir})
+    if(Slicer_BUILD_WEBENGINE_SUPPORT AND EXISTS ${qtwebenginecore_src_dir})
       file(COPY "${qtwebenginecore_src_dir}/Contents" DESTINATION "${qtwebenginecore_dest_dir}/" USE_SOURCE_PERMISSIONS)
       set(qtwebengineprocess_executable "${qtwebenginecore_dest_dir}/Contents/MacOS/QtWebEngineProcess")
       # Collect list of Qt framework dependencies