How do you get the older Mac OSX SDKs for Xcode?

How can I can the 10.6 version of MacOSX SDK with the latest version of the Xcode (version 4.6)?

What version of macOS do I need for Xcode?

Xcode 13 requires a Mac running macOS 11.3 or later.

Where is macOS SDK installed?

The SDK is located at /Applications/Xcode. app/Contents/Developer/Platforms/MacOSX. platform/Developer/SDKs/MacOSX.

You would have to install an older Xcode, take MacOSX10.6.sdk out of the SDK folder at the location it's installed (default: /Developer/Platforms/MacOSX.platform/Developer/SDKs) and copy it to the SDK folder in your latest Xcode.app (default: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs).

As for fixing your error, try importing/including <sys/ucontext.h> instead of Apple's ucontext.h.