RFR: 8179915: Update extLink taglet to allow not linking the description

[Kumar Srinivasan]

Hi,

Please review fix for the custom taglet to allow openjdk builds, in 
which case
the property "build.tools.taglet.ExtLink.NO_LINK" must be set to inhibit 
server
side lookups.

Thanks
Kumar

Webrev: http://cr.openjdk.java.net/~ksrini/8179915/webrev.0
JBS: https://bugs.openjdk.java.net/browse/JDK-8179915