cmake: set default CMAKE_OSX_DEPLOYMENT_TARGET before project()

project() appears to set it to an empty string (the value of nonexistent
$ENV{MACOSX_DEPLOYMENT_TARGET}?), causing our attempt to set its default
to fail. CMake bug?

CMakeLists.txt

@@ -9,6 +9,7 @@ set(CMAKE_POLICY_DEFAULT_CMP0069 NEW)
 
 set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake" ${CMAKE_MODULE_PATH})
 set(CMAKE_USER_MAKE_RULES_OVERRIDE "${CMAKE_CURRENT_SOURCE_DIR}/cmake/DefaultBuildFlags.cmake")
+set(CMAKE_OSX_DEPLOYMENT_TARGET "10.15" CACHE STRING "Minimum OS X deployment version")
 
 option(USE_VCPKG "Use vcpkg for dependency packages" OFF)
 if (USE_VCPKG)
@@ -29,8 +30,6 @@ include(CheckIPOSupported)
 include(SetupCCache)
 include(Sanitizers)
 
-set(CMAKE_OSX_DEPLOYMENT_TARGET "10.15" CACHE STRING "Minimum OS X deployment version")
-
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_STANDARD 17)

flake.nix

@@ -95,7 +95,13 @@
             libtool
             ninja
             pkg-config
+            python3
           ];
+
+          # Undo the SDK setup done by nixpkgs so we can use AppleClang
+          shellHook = ''
+            unset DEVELOPER_DIR SDKROOT MACOSX_DEPLOYMENT_TARGET
+         '';
         };
       };
     }