CMake: Propagate minimum required C++ standard to consumers of Qt

Aka the version of C++ that needs to be supported when compiling
applications that use Qt headers (C++17 at the moment).

Change-Id: I64dec297e8329f31b1d9864f216a95782049ed06
Reviewed-by: Leander Beernaert <leander.beernaert@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtBaseGlobalTargets.cmake

@@ -200,6 +200,10 @@ target_include_directories(GlobalConfigPrivate INTERFACE
 )
 add_library(Qt::GlobalConfigPrivate ALIAS GlobalConfigPrivate)
 
+# Propagate minimum C++ 17 via Platform to Qt consumers (apps), after the global features
+# are computed.
+qt_set_language_standards_interface_compile_features(Platform)
+
 # defines PlatformCommonInternal PlatformModuleInternal PlatformPluginInternal PlatformToolInternal
 include(QtInternalTargets)
 

cmake/QtBuild.cmake

@@ -4075,6 +4075,13 @@ function(qt_set_language_standards)
     endif()
 endfunction()
 
+function(qt_set_language_standards_interface_compile_features target)
+    # Regardless of which C++ standard is used to build Qt itself, require C++17 when building
+    # Qt applications using CMake (because the Qt header files use C++17 features).
+    set(cpp_feature "cxx_std_17")
+    target_compile_features("${target}" INTERFACE ${cpp_feature})
+endfunction()
+
 function(qt_enable_msvc_cplusplus_define target visibility)
     # For MSVC we need to explicitly pass -Zc:__cplusplus to get correct __cplusplus.
     # Check qt_config_compile_test for more info.