RFR: JDK-8215635: Pandoc check in Docs.gmk does not work on Windows

[Tim Bell]

Erik:

> In JDK-8214910 a check for if the PANDOC variable had a value was added
> to Docs.gmk. This check does not work correctly on Windows where PANDOC
> will always have the fixpath prefix. This patch adds a separate
> ENABLE_PANDOC variable that make can check.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8215635
>
> Webrev: http://cr.openjdk.java.net/~erikj/8215635/webrev.01/

Looks good.

Tim