RFR: JDK-8243109: Bootcycle build failures after Nashorn removal

[Erik Joelsson]

With Nashorn removed, the bootcycle build is now broken for builds that 
process markdown files for docs or man pages. Same goes for building 
with with a recent enough JDK 15 as your bootjdk. While a long term 
solution is being worked on, we need something to keep our CI builds 
from failing. For now that would simply be disabling these parts of the 
build if a jjs launcher cannot be found in the boot jdk.

This patch checks the bootjdk for jjs. If not found, ENABLE_PANDOC is 
set to false. This will in turn prevent full docs from being enabled. 
For local bootcycle builds, I explicitly disable pandoc in 
bootcycle-spec.gmk.in. For the jib profile linux-x64-bootcycle-prebuilt, 
the "--enable-full-docs" configure arg is inherited from the main 
profile, so needed to neutralize that with an override.

I had to reorder a little bit in configure.ac to have the bootjdk setup 
when processing JDK options.

Webrev: http://cr.openjdk.java.net/~erikj/8243109/webrev.01/index.html

Bug: https://bugs.openjdk.java.net/browse/JDK-8243109

/Erik