CMake: Don't propagate -fapplication-extension to user projects

Both the compiler and linker -fapplication-extension flag should only
be applied when building Qt's libraries (not executables).

It's up to the user project whether their code will be restricted with
application-extension-only APIs.

In qmake that can be achieved by adding to the qmake project
  CONFIG += app_extension_api_only

In CMake it can be achieved by either adding the compiler and link flags
in the project directly (using target_X_options) or by setting the
appropriate setting in the Xcode project when using the Xcode
generator.

Amends e189126f1a

Pick-to: 6.2
Task-number: QTBUG-95199
Change-Id: Ie7a764d460a89c7650391abff0fcc5abfcabef64
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

cmake/Qt3rdPartyLibraryHelpers.cmake

@@ -50,6 +50,7 @@ function(qt_internal_add_common_qt_library_helper target)
     endif()
 
     _qt_internal_add_library(${target} ${arg_STATIC} ${arg_SHARED} ${arg_MODULE} ${arg_INTERFACE})
+    qt_internal_mark_as_internal_library(${target})
 endfunction()
 
 # Wrapper function to create a regular cmake target and forward all the

cmake/QtInternalTargets.cmake

@@ -325,8 +325,22 @@ function(qt_handle_apple_app_extension_api_only)
         # Build Qt libraries with -fapplication-extension. Needed to avoid linker warnings
         # transformed into errors on darwin platforms.
         set(flags "-fapplication-extension")
-        set(genex_condition "$<NOT:$<BOOL:$<TARGET_PROPERTY:QT_NO_APP_EXTENSION_ONLY_API>>>")
-        set(flags "$<${genex_condition}:${flags}>")
+
+        # The flags should only be applied to internal Qt libraries like modules and plugins.
+        # The reason why we use a custom property to apply the flags is because there's no other
+        # way to prevent the link options spilling out into user projects if the target that links
+        # against PlatformXInternal is a static library.
+        # The exported static library's INTERFACE_LINK_LIBRARIES property would contain
+        # $<LINK_ONLY:PlatformXInternal> and PlatformXInternal's INTERFACE_LINK_OPTIONS would be
+        # applied to a user project target.
+        # So to contain the spilling out of the flags, we ensure the link options are only added
+        # to internal Qt libraries that are marked with the property.
+        set(not_disabled "$<NOT:$<BOOL:$<TARGET_PROPERTY:QT_NO_APP_EXTENSION_ONLY_API>>>")
+        set(is_qt_internal_library "$<BOOL:$<TARGET_PROPERTY:_qt_is_internal_library>>")
+
+        set(condition "$<AND:${not_disabled},${is_qt_internal_library}>")
+
+        set(flags "$<${condition}:${flags}>")
         target_compile_options(PlatformModuleInternal INTERFACE ${flags})
         target_link_options(PlatformModuleInternal INTERFACE ${flags})
         target_compile_options(PlatformPluginInternal INTERFACE ${flags})

cmake/QtModuleHelpers.cmake

@@ -105,6 +105,7 @@ function(qt_internal_add_module target)
     endif()
 
     _qt_internal_add_library("${target}" ${type_to_create})
+    qt_internal_mark_as_internal_library("${target}")
 
     get_target_property(target_type ${target} TYPE)
 

cmake/QtPluginHelpers.cmake

@@ -76,6 +76,7 @@ function(qt_internal_add_plugin target)
             ${ARGN}
     )
     qt6_add_plugin(${target} ${plugin_args})
+    qt_internal_mark_as_internal_library(${target})
 
     qt_get_sanitized_plugin_type("${arg_TYPE}" plugin_type_escaped)
 

cmake/QtTargetHelpers.cmake

@@ -753,3 +753,12 @@ function(qt_internal_adjust_main_config_runtime_output_dir target output_dir)
         RUNTIME_OUTPUT_DIRECTORY_${main_cmake_configuration} "${output_dir}"
     )
 endfunction()
+
+# Marks a target with a property that it is a library (shared or static) which was built using the
+# internal Qt API (qt_internal_add_module, qt_internal_add_plugin, etc) as opposed to it being
+# a user project library (qt_add_library, qt_add_plugin, etc).
+#
+# Needed to allow selectively applying certain flags via PlatformXInternal targets.
+function(qt_internal_mark_as_internal_library target)
+    set_target_properties(${target} PROPERTIES _qt_is_internal_library TRUE)
+endfunction()