macOS 10.4: Guard non-existing APIs

Based on @AJenbo's patch.

CMake/Definitions.cmake

@@ -90,6 +90,8 @@ foreach(
   REMAP_KEYBOARD_KEYS
   DEVILUTIONX_DEFAULT_RESAMPLER
   STREAM_ALL_AUDIO_MIN_FILE_SIZE
+  DARWIN_MAJOR_VERSION
+  DARWIN_MINOR_VERSION
 )
   if(DEFINED ${def_name} AND NOT ${def_name} STREQUAL "")
     list(APPEND DEVILUTIONX_DEFINITIONS ${def_name}=${${def_name}})

CMake/Platforms.cmake

@@ -75,3 +75,17 @@ endif()
 if(NXDK)
   include(platforms/xbox_nxdk)
 endif()
+
+if(CMAKE_SYSTEM_NAME MATCHES "Darwin")
+  # Some notable Darwin (macOS kernel) versions are:
+  #   8.x == macOS 10.4 (Tiger)
+  #   9.x == macOS 10.5 (Leopard)
+  #
+  # Importantly, a lot of the APIs first appeared in version 9, including
+  # the feature availability API (the <Availability.h> header).
+  #
+  # For Darwin 8 and below, we have to rely on the kernel version
+  # to detect available APIs.
+  string(REGEX REPLACE "^([0-9]+)\\.([0-9]+).*$" "\\1" DARWIN_MAJOR_VERSION "${CMAKE_SYSTEM_VERSION}")
+  string(REGEX REPLACE "^([0-9]+)\\.([0-9]+).*$" "\\2" DARWIN_MINOR_VERSION "${CMAKE_SYSTEM_VERSION}")
+endif()

Source/platform/locale.cpp

@@ -161,7 +161,7 @@ std::vector<std::string> GetLocales()
 		locales.push_back(std::move(locale));
 	}
 #endif
-#elif defined(__APPLE__) && defined(USE_COREFOUNDATION)
+#elif defined(__APPLE__) && defined(USE_COREFOUNDATION) && DARWIN_MAJOR_VERSION >= 9
 	// Get the user's language list (in order of preference)
 	CFArrayRef languages = CFLocaleCopyPreferredLanguages();
 	CFIndex numLanguages = CFArrayGetCount(languages);

Source/utils/file_util.cpp

@@ -36,7 +36,7 @@
 #include <unistd.h>
 #endif
 
-#ifdef __APPLE__
+#if defined(__APPLE__) && DARWIN_MAJOR_VERSION >= 9
 #include <copyfile.h>
 #endif
 
@@ -346,7 +346,7 @@ void CopyFileOverwrite(const char *from, const char *to)
 	if (!::CopyFileW(&fromUtf16[0], &toUtf16[0], /*bFailIfExists=*/false)) {
 		LogError("Failed to copy {} to {}", from, to);
 	}
-#elif defined(__APPLE__)
+#elif defined(__APPLE__) && DARWIN_MAJOR_VERSION >= 9
 	::copyfile(from, to, nullptr, COPYFILE_ALL);
 #elif defined(DVL_HAS_FILESYSTEM)
 	std::error_code error;

Source/utils/stdcompat/filesystem.hpp

@@ -1,6 +1,6 @@
 #pragma once
 
-#if defined(__APPLE__)
+#if defined(__APPLE__) && DARWIN_MAJOR_VERSION >= 9
 #include <Availability.h>
 #if (defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && __MAC_OS_X_VERSION_MIN_REQUIRED < 101500) \
     || (defined(__IPHONE_OS_VERSION_MIN_REQUIRED) && __IPHONE_OS_VERSION_MIN_REQUIRED < 130000)