qmake: Add support for CONFIG += c++17

The correct name is c++1z. Anyhow, this is easy enough to get wrong,
so make sure CONFIG += c++17 works as well.

Task-number: QTBUG-67527
Change-Id: Iea26b18824b38b1b5170f85987cf5c750b8e10ab
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: André Hartmann <aha_1980@gmx.de>

mkspecs/features/default_post.prf

@@ -113,6 +113,8 @@ breakpad {
     !isEmpty(QMAKE_STRIP):QMAKE_POST_LINK = $$QMAKE_POST_LINK$$escape_expand(\\n\\t)$$quote($$QMAKE_STRIP $$DEBUGFILENAME)
 }
 
+c++17: CONFIG += c++1z
+
 !c++11:!c++14:!c++1z {
     # Qt requires C++11 since 5.7, check if we need to force a compiler option
     QT_COMPILER_STDCXX_no_L = $$replace(QT_COMPILER_STDCXX, "L$", "")

qmake/doc/src/qmake-manual.qdoc

@@ -973,6 +973,7 @@
     \row    \li c++1z \li C++17 support is enabled. This option has no effect if
             the compiler does not support C++17, or can't select the C++ standard.
             By default, support is disabled.
+    \row    \li c++17 \li Same as c++1z.
     \row    \li strict_c++ \li Disables support for C++ compiler extensions.
             By default, they are enabled.
     \row    \li depend_includepath \li Appending the value of INCLUDEPATH to