GH-95283: Add note about compilers in Mac/README.txt (#99506)

The build machinery assumes that the compiler that's used
to build on macOS includes an SDK that's at least as new
as the OS version on the build machine. Explicitly mention
this in Mac/README.txt.

Mac/README.rst

@@ -10,6 +10,19 @@ Python on macOS README
 This document provides a quick overview of some macOS specific features in
 the Python distribution.
 
+Compilers for building on macOS
+===============================
+
+The core developers primarily test builds on macOS with Apple's compiler tools,
+either Xcode or the Command Line Tools.  For these we only support building with
+a compiler that includes an SDK that targets the OS on the build machine, that is
+the version of Xcode that shipped with the OS version or one newer.
+
+For example, for macOS 12 we support Xcode 13 and Xcode 14 (or the corresponding
+Command Line Tools).
+
+Building with other compilers, such as GCC, likely works, but is not actively supported.
+
 macOS specific arguments to configure
 =====================================