Issue #25136: Add reference to 'xcode-select --install' to Mac README.
diff --git a/Mac/README b/Mac/README
index d9622a6..6e08fdc 100644
--- a/Mac/README
+++ b/Mac/README
@@ -46,7 +46,12 @@
The optional argument specifies which OS X SDK should be used to perform the
build. If xcodebuild is available and configured, this defaults to
the Xcode default MacOS X SDK, otherwise ``/Developer/SDKs/MacOSX.10.4u.sdk``
- if available or ``/`` if not.
+ if available or ``/`` if not. When building on OS X 10.5 or later, you can
+ specify ``/`` to use the installed system headers rather than an SDK. As of
+ OS X 10.9, you should install the optional system headers from the Command
+ Line Tools component using ``xcode-select``::
+
+ $ sudo xcode-select --install
See the section _`Building and using a universal binary of Python on Mac OS X`
for more information.