SDL: Added documentation to fix building on macOS with Xcode 12.2 (thanks @buhman!)

From 6466c10ae4eb11c961d2d97e32c2fac9d3f0b1c6 Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Wed, 1 Apr 2026 16:37:14 -0700
Subject: [PATCH] Added documentation to fix building on macOS with Xcode 12.2
 (thanks @buhman!)

Fixes https://github.com/libsdl-org/SDL/issues/15167
---
 docs/README-macos.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/docs/README-macos.md b/docs/README-macos.md
index 0a97b3d5b9d4f..79b3a4f868dd2 100644
--- a/docs/README-macos.md
+++ b/docs/README-macos.md
@@ -32,6 +32,10 @@ sudo cmake --install .
 
 Please note that building SDL requires at least Xcode 12.2 and the macOS 11.0 SDK.
 
+If you are getting errors building SDL_mfijoystick.m with Xcode 12.2, find your SDKs
+directory and move MacOSX10.15.sdk out of the way so it isn't accidentally being used
+by the build environment.
+
 To use the library once it's built, you essential have two possibilities:
 use the traditional autoconf/automake/make method, or use Xcode.