The JDK build will try to build manual pages if pandoc is present.

However that is not sufficient since you also need to check for
support for the required format.

JDK uses markdown-smart which isn't present on Ubuntu 16.04 LTS
which has pandoc 1.16.0.2
$ pandoc -v
pandoc 1.16.0.2

$ pandoc -f markdown-smart
pandoc: Unknown extension: smart

It isn't clear to me that there is a way to install an extension
since the man page for pandoc lists all the supported formats.
So unless I am missing something this breaks the default build
on Linux for many folks.
Note : I have looked to see if this has been reported and I saw discussion but no bug.