[macOS] Build infrastructure and portability fixes

Build system:
- Enable ObjC++, macOS deployment target, framework linking
- Fix AppleClang detection
- macOS platform name for output directories
- Skip Vulkan on macOS, null graphics without Vulkan dependency
- macOS SDL2 framework detection and discord-rpc macOS sources

Portability fixes:
- mach_absolute_time for macOS tick counting
- _NSGetExecutablePath for macOS
- guard MAP_FIXED_NOREPLACE, ftruncate64->ftruncate
- guard Vulkan include/usage, fix data root for all POSIX

src/xenia/gpu/null/CMakeLists.txt

@@ -1,7 +1,11 @@
 add_library(xenia-gpu-null STATIC)
 xe_platform_sources(xenia-gpu-null ${CMAKE_CURRENT_SOURCE_DIR})
-target_include_directories(xenia-gpu-null PRIVATE
-  ${PROJECT_SOURCE_DIR}/third_party/Vulkan-Headers/include
-)
-target_link_libraries(xenia-gpu-null PUBLIC xenia-base xenia-gpu xenia-ui xenia-ui-vulkan xxhash)
+if(APPLE)
+  target_link_libraries(xenia-gpu-null PUBLIC xenia-base xenia-gpu xenia-ui xxhash)
+else()
+  target_include_directories(xenia-gpu-null PRIVATE
+    ${PROJECT_SOURCE_DIR}/third_party/Vulkan-Headers/include
+  )
+  target_link_libraries(xenia-gpu-null PUBLIC xenia-base xenia-gpu xenia-ui xenia-ui-vulkan xxhash)
+endif()
 xe_target_defaults(xenia-gpu-null)

src/xenia/gpu/null/null_graphics_system.cc

@@ -10,7 +10,9 @@
 #include "xenia/gpu/null/null_graphics_system.h"
 
 #include "xenia/gpu/null//null_command_processor.h"
+#if !XE_PLATFORM_MAC
 #include "xenia/ui/vulkan/vulkan_provider.h"
+#endif
 #include "xenia/xbox.h"
 
 namespace xe {
@@ -25,9 +27,13 @@ X_STATUS NullGraphicsSystem::Setup(cpu::Processor* processor,
                                    kernel::KernelState* kernel_state,
                                    ui::WindowedAppContext* app_context,
                                    bool with_presentation) {
+#if XE_PLATFORM_MAC
+  provider_ = nullptr;
+#else
   // This is a null graphics system, but we still setup vulkan because UI needs
   // it through us :|
   provider_ = xe::ui::vulkan::VulkanProvider::Create(false, with_presentation);
+#endif
   return GraphicsSystem::Setup(processor, kernel_state, app_context,
                                with_presentation);
 }