It looks like MacOSX11.3.sdk shipped with some missing headers by Xcode 12.5.
I downloaded MacOSX11.1.sdk.tar.xz from https://github.com/phracker/MacOSX-SDKs/releases and set my SDKROOT environment variable, and was able to compile the simple program.
Please update a new SDK :)
xerothermic
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution