doc: recommend building Go 1.4 with CGO_ENABLED=0

Fixes #21054

Change-Id: I016486dc62c04a80727f8da7d1dcec52f2c7f344
Reviewed-on: https://go-review.googlesource.com/62291Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/install-source.html

@@ -148,7 +148,8 @@ which contains the Go 1.4 source code plus accumulated fixes
 to keep the tools running on newer operating systems.
 (Go 1.4 was the last distribution in which the tool chain was written in C.)
 After unpacking the Go 1.4 source, <code>cd</code> to
-the <code>src</code> subdirectory and run <code>make.bash</code> (or,
+the <code>src</code> subdirectory, set <code>CGO_ENABLED=0</code> in
+the environment, and run <code>make.bash</code> (or,
 on Windows, <code>make.bat</code>).
 </p>