Require the C++17 standard for configure-time syncqt

Require the C++17 standard, but not what is set by Qt for syncqt,
to avoid unwanted standard check in some corner cases.

Pick-to: 6.5 6.6
Task-number: QTBUG-116789
Change-Id: I34e07e172e31d5ffc69b84dd3ef30f02f6e75a9d
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/tools/syncqt/CMakeLists.txt

@@ -60,7 +60,7 @@ else()
         INSTALL_DIRECTORY "${INSTALL_LIBEXECDIR}"
         CMAKE_FLAGS
             -DCMAKE_CXX_STANDARD_REQUIRED:BOOL=TRUE
-            -DCMAKE_CXX_STANDARD:STRING=${CMAKE_CXX_STANDARD}
+            -DCMAKE_CXX_STANDARD:STRING=17
             # std::filesystem API is only available in macOS 10.15+
             -DCMAKE_OSX_DEPLOYMENT_TARGET:STRING=10.15
             "${osx_architectures}"