Shared compilation cache enabled on try (sort of)

[Mike Hommey]

Hi,

In an effort to make build turnaround times better, we're working on
various things, on one which is the use of a shared compilation cache
instead of a per-slave ccache. The first milestone for this project is
reached, and this shared cache is now enabled for try builds for desktop
linux[0] builds, as long as they include the required mozconfig and
tooltool manifest changes, which I just landed on mozilla-central.

Please rebase your future try pushes on top of current mozilla-central
(or mozilla-inbound or your favorite project branch once m-c will have
been merged there). The changeset to look for in your history is
a62bde1d6efe.

In the event that something is broken on your try push because of this,
please first ensure this is really the cause of your problem by disabling
the shared cache[1], and if it turns out to be it, please file a bug a
make it block bug 940788.

Other branches and other platforms will follow, including some that
currently don't even use ccache for some reason.

If you are interested in some analysis associated with the use of that
cache vs. ccache, you can read about the experiments I run during the
past several weeks:
http://glandium.org/blog/?p=3054
http://glandium.org/blog/?p=3079
(There's also a third post to be published either today or tomorrow ;
it will be on planet mozilla as usual, and on my blog, directly)

Cheers,

Mike

0. linux and linux64, opt and debug.
1. Reverting part of a62bde1d6efe is enough for this. e.g.
hg log -pr a62bde1d6efe browser/config/tooltool-manifests | patch -p1 -R