RFR: JDK-8217032: Check pandoc capabilities in configure

[Erik Joelsson]

Please review this minor fix. The JDK build now has support for 
generating man pages, and will try to do so if it finds pandoc on the 
system. Unfortunately, not all versions of pandoc are valid, and if a 
bad version is found, the build will fail. This patch adds an extensions 
check for the found pandoc in configure, and if the "smart" extension 
that we use is missing, pandoc is disabled.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217032
Webrev: http://cr.openjdk.java.net/~erikj/8217032/webrev.01/

/Erik