update Windows and Mac OS build instructions

Explain how to deal with the "racket-lib" that is normally included
with a source distribution, but that doesn't have dependencies
specific to Windows or Mac OS.

racket/src/README.txt

@@ -285,6 +285,17 @@ First, install Xcode and command-line tools from Apple.
 After installing developer tools, follow the Unix instructions above,
 but note the following:
 
+ * If you are building from a source distribution (as opposed to a Git
+   repository checkout), then most likely "racket-lib" is already
+   included and installed as part of the the distribution, but without
+   dependencies of "racket-lib" that are specific to Mac OS. In that
+   case, use
+
+      raco pkg update --auto racket-lib
+
+   using `raco` as created by `make install` to download and install
+   the dependencies.
+
  * The Racket build creates a framework, "Racket.framework", which is
    installed into "racket/lib".  This framework is used by the `racket`
    executable that goes into "racket/bin".

racket/src/worksp/README.txt

@@ -91,7 +91,16 @@ libraries, finish with
 
    ..\..\raco pkg install racket-lib
 
-See also "..\native-lib\README.txt".
+If you are building from a source distribution (as opposed to a Git
+repository checkout), then most likely "racket-lib" is already
+included and installed as part of the the distribution, but without
+Windows-specific dependencies of "racket-lib". In that case, instead
+of the above command, use
+
+   ..\..\raco pkg update --auto racket-lib
+
+Only if you are starting completely from scratch, see also
+"..\native-lib\README.txt".
 
 
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