Add a separate QT_FEATURE for std::format

We want tst_qfloat16 to be built in C++20 mode, even if Qt itself is
not built in C++20 mode, which means we can't use QT_FEATURE_cxx20 as
check in tst_qfloat16's CMakeLists.txt.

In addition, even if the compiler supports C++20, the standard library
may not support all features we need. Specifically, due to the
deployment target of macOS being 12 we can't rely on std::to_chars
being available, as is only available since macOS 13.4.

This patch introduces a separate QT_FEATURE_cxx20_format, and
adjusts the tst_qfloat16's CMakeLists.txt to use this feature.

Note that we intentionally do not add QT_FEATURE_cxx20 as a
hard condition to the new check, because it might be disabled
during the Qt configuration, but we still want the test to be
built in C++20 mode.

Note also, that we cannot use the QT_CONFIG(cxx20_format)
check in the qtformat_impl.h header, because the std::format
support is header-only, and user project might be compiled
for a different minimal macOS version, so we do not want to
rely on the Qt-specific config.

Pick-to: 6.9 6.8
Change-Id: Ibc43d243dbb24fcb922647fe2d90f61491144eb7
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/configure.cmake

@@ -552,6 +552,28 @@ int main(void)
 }
 ")
 
+# std::format support
+qt_config_compile_test(cxx20_format
+    LABEL "C++20 std::format support"
+    CODE
+"#include <format>
+#include <string>
+
+#if !defined(__cpp_lib_format) || (__cpp_lib_format < 202106L)
+#error
+#endif
+
+int main(void)
+{
+    /* BEGIN TEST: */
+const auto s = std::format(\"{}\", 1);
+    /* END TEST: */
+    return 0;
+}
+"
+    CXX_STANDARD 20
+)
+
 # <stacktrace>
 qt_config_compile_test(cxx23_stacktrace
     LABEL "C++23 <stacktrace> support"
@@ -849,6 +871,11 @@ qt_feature("backtrace" PRIVATE
     LABEL "backtrace"
     CONDITION UNIX AND QT_FEATURE_regularexpression AND WrapBacktrace_FOUND
 )
+qt_feature("cxx20_format" PRIVATE
+    LABEL "C++20 std::format support"
+    CONDITION TEST_cxx20_format # intentionally not checking QT_FEATURE_cxx20!
+    AUTODETECT TRUE
+)
 qt_feature("cxx23_stacktrace" PRIVATE
     LABEL "C++23 <stacktrace>"
     CONDITION TEST_cxx23_stacktrace AND QT_FEATURE_cxx2b

tests/auto/corelib/global/qfloat16/CMakeLists.txt

@@ -19,7 +19,7 @@ qt_internal_add_test(tst_qfloat16
 )
 
 # To test std::format support, if possible
-if(NOT VXWORKS)
+if(QT_FEATURE_cxx20_format)
     set_target_properties(tst_qfloat16
         PROPERTIES
             CXX_STANDARD 20