Update to SDL 2.24.0 and add separate macOS UB2 dylib

Update mingw and macOS SDL libraries to 2.24.0. (MSVC .lib files and
Xcode project haven't been updated.)

There is now a separate SDL dylib for macOS Universal Binary 1 and 2
because SDL 2.24.0 dropped support for macOS 10.6 (which is last to
support x86). Documentation says SDL 2.24.0 also dropped macOS 10.7
and 10.8 too. So UB2 is now 10.9 or later while 10.5 to 10.8 live in
UB1.

Compiled using osxcross with macOS 11.3 SDK. Ran "strip -x" on Windows
and macOS libraries. Ad-hoc signed dylib using PyOxidizer rcodesign to
fix digest checksums after strip.

Makefile

@@ -294,10 +294,31 @@ ifeq ($(PLATFORM),darwin)
 
   # Default minimum Mac OS X version
   ifeq ($(MACOSX_VERSION_MIN),)
-    MACOSX_VERSION_MIN=10.7
+    MACOSX_VERSION_MIN=10.9
     ifneq ($(findstring $(ARCH),ppc ppc64),)
       MACOSX_VERSION_MIN=10.5
     endif
+    ifeq ($(ARCH),x86)
+      MACOSX_VERSION_MIN=10.6
+    endif
+    ifeq ($(ARCH),x86_64)
+      # trying to find default SDK version is hard
+      # macOS 10.15 requires -sdk macosx but 10.11 doesn't support it
+      # macOS 10.6 doesn't have -show-sdk-version
+      DEFAULT_SDK=$(shell xcrun -sdk macosx -show-sdk-version 2> /dev/null)
+      ifeq ($(DEFAULT_SDK),)
+        DEFAULT_SDK=$(shell xcrun -show-sdk-version 2> /dev/null)
+      endif
+      ifeq ($(DEFAULT_SDK),)
+        $(error Error: Unable to determine macOS SDK version.  On macOS 10.6 to 10.8 run: make MACOSX_VERSION_MIN=10.6  On macOS 10.9 or later run: make MACOSX_VERSION_MIN=10.9 );
+      endif
+
+      ifneq ($(findstring $(DEFAULT_SDK),10.6 10.7 10.8),)
+        MACOSX_VERSION_MIN=10.6
+      else
+        MACOSX_VERSION_MIN=10.9
+      endif
+    endif
     ifeq ($(ARCH),arm64)
       MACOSX_VERSION_MIN=11.0
     endif