- Remove c++11 flag from CONFIG (added by mkspecs/common/msvc-version.conf for every Visual Studio > 2012) when there is c++14, c++17 or c++20, in order to get homogeneous rules in our template pri files

- Add c++11 error message handling for msvc : conan compiler.cppstd setting doesn't support value = 11
- Add c++20 support for conan compiler.cppstd setting

Author: Firefly35

bcom_arch_define.pri

@@ -99,6 +99,11 @@ isEmpty(BCOM_TARGET_PLATFORM) {
                 # Note : msvc version set to 14.1 for Visual Studio 2017 in order to separate version from msvc 2015!
                 BCOM_COMPILER_VER = 14.1
             }
+        }
+        contains(CONFIG,c++14)|contains(CONFIG,c++1z)|contains(CONFIG,c++17)|contains(CONFIG,c++2a)|contains(CONFIG,c++20) {
+            contains(CONFIG,c++11) {
+                CONFIG -= c++11
+            }
         }
 		# note : when icl is used with msvc, the most important is the msvc compiler version!
         BCOM_TARGET_PLATFORM = win-$$basename(QMAKE_CC)-$$BCOM_COMPILER_VER

packagedependencies.pri

@@ -300,14 +300,21 @@ QMAKE_OBJECTIVE_CFLAGS += $${QMAKE_CXXFLAGS}
     }
     write_file($$_PRO_FILE_PWD_/build/conanfile.txt, CONANFILECONTENT)
     contains(CONFIG,c++11) {
-        conanCppStd=11
+        !msvc {
+            conanCppStd=11
+        } else {
+            error("Invalid setting for conan : compiler.cppstd supported values for msvc are 14, 17, 20")
+        }
     }
     contains(CONFIG,c++14) {
         conanCppStd=14
     }
     contains(CONFIG,c++1z)|contains(CONFIG,c++17) {
         conanCppStd=17
     }
+    contains(CONFIG,c++2a)|contains(CONFIG,c++20) {
+        conanCppStd=20
+    }
     CONFIG += conan_basic_setup
 #conan install -o boost:shared=True -s build_type=Release -s cppstd=14 boost/1.68.0@conan/stable
     system(conan install $$_PRO_FILE_PWD_/build/conanfile.txt -s compiler.cppstd=$${conanCppStd} -s build_type=$${CONANBUILDTYPE} --build=missing -if $$_PRO_FILE_PWD_/build)