GH-95283: Add note about compilers in Mac/README.txt (GH-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.
(cherry picked from commit 8f024a02d7d63315ecc3479f0715e927f48fc91b)

Co-authored-by: Ronald Oussoren <ronaldoussoren@mac.com>