Add support for c++ 20 also with gcc 9

Author: ChristianFeldmann

YUViewApp/YUViewApp.pro

@@ -2,9 +2,14 @@ QT += core gui widgets opengl xml concurrent network
 
 TARGET = YUView
 TEMPLATE = app
-CONFIG += c++20
 CONFIG -= debug_and_release
 
+CONFIG += c++20
+gcc {
+  # For gcc 9, setting 20 does not work. Must set c++2a.
+  equals(QMAKE_GCC_MAJOR_VERSION, 9): QMAKE_CXXFLAGS += -std=c++2a
+}
+
 SOURCES += $$files(src/*.cpp, false)
 HEADERS += $$files(src/*.h, false)
 

YUViewLib/YUViewLib.pro

@@ -2,10 +2,15 @@ QT += core gui widgets opengl xml concurrent network
 
 TEMPLATE = lib
 CONFIG += staticlib
-CONFIG += c++20
 CONFIG -= debug_and_release
 CONFIG += object_parallel_to_source
 
+CONFIG += c++20
+gcc {
+  # For gcc 9, setting 20 does not work. Must set c++2a.
+  equals(QMAKE_GCC_MAJOR_VERSION, 9): QMAKE_CXXFLAGS += -std=c++2a
+}
+
 SOURCES += $$files(src/*.cpp, true)
 HEADERS += $$files(src/*.h, true)
 

YUViewUnitTest/YUViewUnitTest.pro

@@ -6,7 +6,12 @@ TEMPLATE = app
 CONFIG += console
 CONFIG -= app_bundle
 CONFIG -= debug_and_release
+
 CONFIG += c++20
+gcc {
+  # For gcc 9, setting 20 does not work. Must set c++2a.
+  equals(QMAKE_GCC_MAJOR_VERSION, 9): QMAKE_CXXFLAGS += -std=c++2a
+}
 
 SOURCES += $$files(*.cpp, true)
 HEADERS += $$files(*.h, true)