RFR: JDK-8142983: Enable builds with icecc/icecream

[Erik Joelsson]

We should enable builds using icecc/icecream distributed native 
compilation. The setup is rather simple and the required changes to 
configure are no more complex than how we currently handle ccache.

I have done some experiments and it seems to be working quite well. I 
can easily get build times of less than 2 minutes for "make 
exploded-image" using JOBS=100 to fully saturate a couple of extra hosts 
in the cluster. (I need to combine this with --enable-javac-server to 
get the java compilation to not be a bottleneck.)

This is likely not that interesting to automated, already distributed 
build systems, but for a developers day to day work, being attached to 
an icecream cluster could greatly reduce local compile times.

Icecream supports both linux and macosx but I have only tested this on 
linux myself. The implementation in configure will support both gcc and 
clang.

To use the feature, given that you have icecc installed and setup to 
join a cluster, make sure icecc is on the path and simply configure with 
--enable-icecc and it should all work automatically. Configure will 
generate the necessary compiler build environments that get sent to the 
build slaves and setup the needed make variables.

When enabling icecc, precompiled header is disabled. It is possible to 
get precompiled header to work with icecc, but the performance boost 
gets completely negated (and more) by the extra file transfer times of 
the precompiled header file, at least in my experience.

Bug: https://bugs.openjdk.java.net/browse/JDK-8142983
Webrev: http://cr.openjdk.java.net/~erikj/8142983/webrev.01/

/Erik