Drop build-system support for Qt4 and Python 2

Also increase minimum CMake, GCC, and Clang versions to match
Ubuntu 18.04 LTS. This sets our minimum supported version of the C++
standard to C++17.

CMakeLists.txt

@@ -1,5 +1,5 @@
-# april 2020, the oldest we have to support : Ununtu 16.04 LTS (Xenial)
-cmake_minimum_required(VERSION 3.5.1 FATAL_ERROR)
+# March 2021, the oldest we have to support : Ununtu 18.04 LTS
+cmake_minimum_required(VERSION 3.10.2 FATAL_ERROR)
 
 # policy CMP0072 was introduced with CMake 3.11
 # relates to FindOpenGL module
@@ -17,8 +17,8 @@ project(FreeCAD)
 
 set(PACKAGE_VERSION_NAME "Vulcan")
 set(PACKAGE_VERSION_MAJOR "0")
-set(PACKAGE_VERSION_MINOR "19")
-set(PACKAGE_VERSION_PATCH "16100")
+set(PACKAGE_VERSION_MINOR "20")
+set(PACKAGE_VERSION_PATCH "0")
 set(PACKAGE_VERSION_SUFFIX "dev") # either "dev" for development snapshot or "" (empty string)
 set(FREECAD_VERSION_PATCH "0") # number of patch release (e.g. "4" for the 0.18.4 release)
 

cMake/FreeCAD_Helpers/CompilerChecksAndSetups.cmake

@@ -28,22 +28,18 @@ macro(CompilerChecksAndSetups)
                         OUTPUT_VARIABLE CMAKE_CXX_COMPILER_VERSION)
     endif(CMAKE_COMPILER_IS_GNUCXX AND NOT CMAKE_CXX_COMPILER_VERSION)
 
-    # Enabled C++11 for Freecad 0.17 and later
-    if (FREECAD_VERSION VERSION_GREATER 0.16)
-        set(BUILD_ENABLE_CXX_STD "C++11"  CACHE STRING  "Enable C++ standard")
+    # Enabled C++17 for Freecad 0.20 and later
+        set(BUILD_ENABLE_CXX_STD "C++17"  CACHE STRING  "Enable C++ standard")
         set_property(CACHE BUILD_ENABLE_CXX_STD PROPERTY STRINGS
-                     "C++11"
-                     "C++14"
                      "C++17"
                      "C++20"
         )
 
-        if (CMAKE_COMPILER_IS_GNUCXX AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.7)
-            message(FATAL_ERROR "FreeCAD 0.17 and later requires C++11.  G++ must be 4.7 or later, the used version is ${CMAKE_CXX_COMPILER_VERSION}")
-        elseif(CMAKE_COMPILER_IS_CLANGXX AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.3)
-            message(FATAL_ERROR "FreeCAD 0.17 and later requires C++11.  Clang must be 3.3 or later, the used version is ${CMAKE_CXX_COMPILER_VERSION}")
+        if (CMAKE_COMPILER_IS_GNUCXX AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 7.3)
+            message(FATAL_ERROR "FreeCAD 0.20 and later requires C++17.  G++ must be 7.3 or later, the used version is ${CMAKE_CXX_COMPILER_VERSION}")
+        elseif(CMAKE_COMPILER_IS_CLANGXX AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 6.0)
+            message(FATAL_ERROR "FreeCAD 0.20 and later requires C++17.  Clang must be 6.0 or later, the used version is ${CMAKE_CXX_COMPILER_VERSION}")
         endif()
-    endif(FREECAD_VERSION VERSION_GREATER 0.16)
 
     # Escape the two plus chars as otherwise cmake complains about invalid regex
     if(${BUILD_ENABLE_CXX_STD} MATCHES "C\\+\\+20")

cMake/FreeCAD_Helpers/InitializeFreeCADBuildOptions.cmake

@@ -31,18 +31,8 @@ macro(InitializeFreeCADBuildOptions)
         if (DEFINED ENV{FREECAD_LIBPACK_DIR})
             set(FREECAD_LIBPACK_DIR $ENV{FREECAD_LIBPACK_DIR} CACHE PATH  "Directory of the FreeCAD LibPack")
             message(STATUS "Found libpack env variable: ${FREECAD_LIBPACK_DIR}")
-            if (EXISTS ${FREECAD_LIBPACK_DIR}/lib/Qt5Core.lib)
-                option(BUILD_QT5 "Build with Qt5." ON)
-            else()
-                option(BUILD_QT5 "Build with Qt5." OFF)
-            endif()
         else()
             set(FREECAD_LIBPACK_DIR ${CMAKE_SOURCE_DIR} CACHE PATH  "Directory of the FreeCAD LibPack")
-            if (EXISTS ${FREECAD_LIBPACK_DIR}/lib/Qt5Core.lib)
-                option(BUILD_QT5 "Build with Qt5." ON)
-            else()
-                option(BUILD_QT5 "Build with Qt5." OFF)
-            endif()
         endif()
 
         set(LIBPACK_FOUND OFF )
@@ -68,10 +58,10 @@ macro(InitializeFreeCADBuildOptions)
             message(STATUS "Visit: https://github.com/apeltauer/FreeCAD/releases/ for Windows libpack downloads.")
         endif()
     else(MSVC)
-        option(BUILD_QT5 "Build with Qt5." OFF)
         option(FREECAD_LIBPACK_USE "Use the LibPack to Build FreeCAD (only Win32 so far)." OFF)
         set(FREECAD_LIBPACK_DIR ""  CACHE PATH  "Directory of the FreeCAD LibPack")
     endif(MSVC)
+    option(BUILD_QT5 "Build with Qt5." ON)
 
     # https://blog.kitware.com/constraining-values-with-comboboxes-in-cmake-cmake-gui/
     set(FREECAD_USE_OCC_VARIANT "Community Edition"  CACHE STRING  "Official OpenCASCADE version or community edition")