qcompilerdetection.h: include <version>

...and update the documentation for how we detect post-C++11 features,
as proposed on the mailing-list:
  https://lists.qt-project.org/pipermail/development/2022-November/043248.html

According to https://en.cppreference.com/w/cpp/compiler_support,
<version> is available from
- libstdc++ from GCC 9
- libc++ from LLVM 7
- MSVC 19.22 = VS 2019 16.2
- AppleClang 10 = Xcode 10.0 beta (10L176w), 10.0 (10A255), 10.1 (10B61)

It is _not_ available on
- GHS 22.1.4
- GCC 8.3.0 (as used by QNX 7.1, but the default is libc++)

Pick-to: 6.4
Task-number: QTBUG-108228
Change-Id: I61e0727573d1b4559228e3f5bd58d73e86a9256e
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qcompilerdetection.h

@@ -453,6 +453,12 @@
 #define QT_HAS_INCLUDE(x)        __has_include(x)
 #define QT_HAS_INCLUDE_NEXT(x)   __has_include_next(x)
 
+#ifdef __cplusplus
+# if __has_include(<version>) /* remove this check once Integrity, QNX have caught up */
+#  include <version>
+# endif
+#endif
+
 /*
  * C++11 support
  *
@@ -493,10 +499,16 @@
  *  N1653           Q_COMPILER_VARIADIC_MACROS
  *  N2242 N2555     Q_COMPILER_VARIADIC_TEMPLATES       __cpp_variadic_templates = 200704
  *
- * For any future version of the C++ standard, we use only the SD-6 macro.
+ *
+ * For the C++ standards C++14 and C++17, we use only the SD-6 macro.
  * For full listing, see
  *  http://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations
  *
+ * For any future version of the C++ standard, we use only the C++20 feature test macro.
+ * For library features, we assume <version> is present (this header includes it).
+ * For full listing, see
+ *  https://en.cppreference.com/w/User:D41D8CD98F/feature_testing_macros
+ *
  * C++ extensions:
  *  Q_COMPILER_RESTRICTED_VLA       variable-length arrays, prior to __cpp_runtime_arrays
  */