CMake: Set default value for CMAKE_OSX_DEPLOYMENT_TARGET

CMake versions older than 3.19 choose the sysroot based on the deployment target, so this does result in it choosing the 10.9 SDK even when targeting arm64, so we need to work around that.

CMakeLists.txt

@@ -51,6 +51,22 @@ else()
   set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS ${_configs})
 endif()
 
+# Set defaults for macOS, must be before project().
+if(APPLE)
+  set(CMAKE_OSX_DEPLOYMENT_TARGET "10.9" CACHE STRING "Minimum macOS version to target")
+  set(CMAKE_XCODE_ATTRIBUTE_CLANG_CXX_LIBRARY "libc++")
+
+  if(CMAKE_VERSION VERSION_LESS "3.19" AND NOT CMAKE_OSX_SYSROOT)
+    # Older CMake chose SDK based on deployment target, against Apple's recommendations.
+    # However, we need to use the latest to be able to target arm64.
+    if(IS_DIRECTORY "/Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk")
+      set(CMAKE_OSX_SYSROOT "/Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk" CACHE STRING "")
+    elseif(IS_DIRECTORY "/Library/Developer/CommandLineTools/SDKs/MacOSX11.0.sdk")
+      set(CMAKE_OSX_SYSROOT "/Library/Developer/CommandLineTools/SDKs/MacOSX11.0.sdk" CACHE STRING "")
+    endif()
+  endif()
+endif()
+
 # Figure out the version
 set(_s "[\\t ]*") # CMake doesn't support \s*
 file(STRINGS "setup.cfg" _version REGEX "^version${_s}=${_s}")