recent qt6 requires c++20

There are warnings within the qt6 header files if c++20 is not used, for example:

usr/include/x86_64-linux-gnu/qt6/QtCore/qfuturesynchronizer.h:21:5: warning: use of the 'nodiscard' attribute is a C++20 extension [-Wc++20-attribute-extensions]
   21 |     Q_NODISCARD_CTOR_X("Use future.waitForFinished() instead.")
      |     ^
/usr/include/x86_64-linux-gnu/qt6/QtCore/qcompilerdetection.h:972:43: note: expanded from macro 'Q_NODISCARD_CTOR_X'
  972 | #    define Q_NODISCARD_CTOR_X(message) [[nodiscard(message)]]
      |                                           ^
1 warning generated.

qmake6 --version
QMake version 3.1
Using Qt version 6.8.3 in /usr/lib/x86_64-linux-gnu

sys/unix/hints/include/compiler.370

@@ -26,6 +26,7 @@ CCFLAGS = -g
 # If these are set on entry, preparation for C++ compiles is affected.
 #    CPLUSPLUS_NEEDED = 1           C++ compile bits included
 #    CPLUSPLUS_NEED17 = 1           C++ -std=c++17 (at least)
+#    CPLUSPLUS_NEED20 = 1           C++ -std=c++20 (at least)
 #    CPLUSPLUS_NEED_DEPSUPPRESS = 1 C++ -Wno-deprecated-copy,
 #                                       -Wno-deprecated-declarations
 
@@ -149,6 +150,14 @@ else   # g++ version greater than or equal to 12? (no follows)
 CCXX=g++ -std=c++17
 endif  # g++ version greater than or equal to 12
 endif  # CPLUSPLUS_NEED17
+ifdef CPLUSPLUS_NEED20
+ifeq "$(GPPGTEQ9)" "1"
+CCXX=g++ -std=c++20
+else   # g++ version greater than or equal to 9? (no follows)
+CCXX=g++ -std=c++17
+endif  # g++ version greater than or equal to 9
+endif  # CPLUSPLUS_NEED20
+
 
 else   # g++ or clang++ ?
 
@@ -183,6 +192,11 @@ ifeq "$(CLANGPPGTEQ17)" "1"
 CCXX=clang++ -std=c++17
 endif  # clang++ greater than or equal to 17
 endif  # CPLUSPLUS_NEED17
+ifdef CPLUSPLUS_NEED20
+ifeq "$(CLANGPPGTEQ17)" "1"
+CCXX=clang++ -std=c++20
+endif  # clang++ greater than or equal to 17
+endif  # CPLUSPLUS_NEED20
 endif  # end of clang++-specific section
 CXX=$(CCXX)
 endif  # CPLUSPLUS_NEEDED

sys/unix/hints/include/multiw-2.370

@@ -163,7 +163,7 @@ ifndef CPLUSPLUS_NEEDED
 CPLUSPLUS_NEEDED = 1
 endif   # CPLUSPLUS_NEEDED
 ifdef WANT_WIN_QT6
-CPLUSPLUS_NEED17 = 1
+CPLUSPLUS_NEED20 = 1
 CPLUSPLUS_NEED_DEPSUPPRESS = 1
 endif   # WANT_WIN_QT6
 endif   # WANT_WIN_QT