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

[Erik Joelsson]

Hello,

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/

/Erik