pythongh-117886: platform_triplet.c: check if TARGET_OS_* defined

Older macOS SDKs don't define any of these, and some clang versions
will error if you use them without first checking if they are defined.

Author: jmroot

Misc/platform_triplet.c

@@ -246,8 +246,8 @@ PLATFORM_TRIPLET=i386-gnu
 # endif
 #elif defined(__APPLE__)
 #  include "TargetConditionals.h"
-#  if TARGET_OS_IOS
-#    if TARGET_OS_SIMULATOR
+#  if defined(TARGET_OS_IOS) && TARGET_OS_IOS
+#    if defined(TARGET_OS_SIMULATOR) && TARGET_OS_SIMULATOR
 #      if __x86_64__
 PLATFORM_TRIPLET=x86_64-iphonesimulator
 #      else
@@ -256,7 +256,8 @@ PLATFORM_TRIPLET=arm64-iphonesimulator
 #    else
 PLATFORM_TRIPLET=arm64-iphoneos
 #    endif
-#  elif TARGET_OS_OSX
+/* Older macOS SDKs do not define TARGET_OS_OSX */
+#  elif !defined(TARGET_OS_OSX) || TARGET_OS_OSX
 PLATFORM_TRIPLET=darwin
 #  else
 #    error unknown Apple platform