Update build requirements for Apple

CMake version 3.20 or higher (needed to support Xcode project generation)
clang version 12.0 or higher (needed to support arm64+x64 universal binary compilation)
XCode 12.2 is the oldest version which includes clang version 12.0

CMakeLists.txt

@@ -1,6 +1,9 @@
 cmake_minimum_required(VERSION 2.8.12)
 
 if(APPLE)
+  # Building for Apple requires at least CMake 3.20.0
+  cmake_minimum_required(VERSION 3.20.0)
+
   SET(CMAKE_OSX_ARCHITECTURES "x86_64;arm64" CACHE STRING "Build architectures for Mac OS X" FORCE)
   SET(CMAKE_OSX_DEPLOYMENT_TARGET "10.15" CACHE STRING "Minimum SDK for Mac OS X" FORCE)
   SET(CMAKE_XCODE_GENERATE_SCHEME ON)
@@ -194,6 +197,12 @@ option(ENABLE_ASAN "Enable address sanitizer" OFF)
 option(ENABLE_TSAN "Enable thread sanitizer" OFF)
 option(ENABLE_MSAN "Enable memory sanitizer" OFF)
 
+if(APPLE)
+  if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 12.0)
+    message(FATAL_ERROR "Xcode 12.2 or newer is required to build for Apple")
+  endif()
+endif()
+
 if(ENABLE_ASAN)
     set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=address -fno-omit-frame-pointer")
     message(STATUS "Enabling address sanitizer - may cause issues if capturing, only recommended for use with replay only")

docs/CONTRIBUTING/Compiling.md

@@ -27,7 +27,15 @@ Configuration is available for cmake, [documented elsewhere](https://cmake.org/d
 
 First check that you have all of the [required dependencies](Dependencies.md#mac).
 
-Mac support is pretty early and while it will compile, it's not usable for debugging yet and is not officially supported. Builds happen with cmake the same way as Linux.
+Mac support is pretty early and while it will compile, it's not usable for debugging yet and is not officially supported. 
+
+To build with make, use cmake the same way as Linux.
+
+To build with Xcode, use the cmake Xcode generator to create the Xcode project:
+
+```
+cmake -DCMAKE_BUILD_TYPE=Debug -Bbuild -H. -GXcode
+```
 
 ## Android
 

docs/CONTRIBUTING/Dependencies.md

@@ -95,7 +95,7 @@ sudo apt-get install libx11-dev libx11-xcb-dev mesa-common-dev libgl1-mesa-dev l
 
 ## Mac
 
-Mac requires a recent version of CMake, and the same Qt version as the other platforms (at least 5.6.2). If you're using [homebrew](http://brew.sh) then this will do the trick:
+Mac requires Xcode version 12.2 or newer, CMake version 3.20 or newer, and Qt5 version 5.15.2 or newer. If you're using [homebrew](http://brew.sh) then this will do the trick:
 
 ```
 brew install cmake qt5