configure: accept -c++std c++17 again

When 2cf66d49ef removed support for
c++14, it incorrectly also removed support for explicitly setting the
default, C++17. This broke build scripts for no good reason, and made
it impossible to undo an earlier -c++std c++20 again.

Accept C++17 and C++1z again, as an explicit disabler for C++20 and
C++23 features.

Amends 2cf66d49ef.

Fixes: QTBUG-125015
Change-Id: I99b80cd5f41fa5a23259630eae85ef807982f526
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from commit 68e6406623afe5aaaa79e116cbd6b07e8f295e8b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

qt_cmdline.cmake

@@ -141,7 +141,10 @@ function(qt_commandline_cxxstd arg val nextok)
         qtConfAddError("Missing argument to command line parameter '${arg}'.")
         return()
     endif()
-    if(val MATCHES "(c\\+\\+)?(20|2a)")
+    if(val MATCHES "(c\\+\\+)?(17|1z)")
+        qtConfCommandlineDisableFeature(c++20)
+        qtConfCommandlineDisableFeature(c++2b)
+    elseif(val MATCHES "(c\\+\\+)?(20|2a)")
         qtConfCommandlineEnableFeature(c++20)
         qtConfCommandlineDisableFeature(c++2b)
     elseif(val MATCHES "(c\\+\\+)?(2b)")