hg: jdk9/sandbox: 8178725: provide way to link to external documentation

[chris.hegarty at oracle.com]

Changeset: e1cd0e7075ae
Author:    ksrini
Date:      2017-04-25 07:54 -0700
URL:       http://hg.openjdk.java.net/jdk9/sandbox/rev/e1cd0e7075ae

8178725: provide way to link to external documentation
Reviewed-by: dholmes, erikj, ihse, jjg

! make/Javadoc.gmk