dxr-build: allow specifying tree build order

Add support for an optional |order| parameter tp tree configs; it should be
an integer, with smaller values being built first.  Trees with unspecified
order will be built after those with specified order, in sorted by tree
name.
Set LD_LIBRARY_PATH in the json-test makefile. This should solve bug 823322.

I'm not sure where that makefile comes from. If something generates it, we should fix this at the source.
Export compiler command lines in DXR-specific env variables.

Rather than just export CC and CXX environment variables, export the
same information as DXR-specific environment variables.  This helps
integration into non-GNU build systems.
Fix libtrilite not getting built on test.

(Ripped off from abbeyj's pull req)
Merge abbeyj's LD_LIBRARY_PATH fixes in. I like his relative paths better.
Make the tests pass on Jenkins (once it has the Virtualbox kernel module loaded).
Tweak grammar in readme.

Really, I just want to see if Jenkins passes.
Add a comment to the test harness.

This is all going to die soon, but at least I don't have to read the code again. :-)
Add a temporary script that `scp`s the checkout into the Vagrant VM on Jenkins so we can get CI passing.

Once https://bugzilla.mozilla.org/show_bug.cgi?id=826549 is done, we can remove this.
Make tests pass on Jenkins!

* Quit using NFS shared folders on Jenkins, since they don't work at the moment.
* Enable symlinks on the dxr shared folder so trilite will build.
* Make many tweaks to the Jenkins config:
  * Build our vagrant box manually rather than with the Jenkins plugin. Run the tests over ssh from a normal "execute shell" build step as well, since it doesn't find our manually-created box otherwise.
  * Get the path to vagrant executable right.
Stick the few lines of Jenkins shell script in a file so we can change-track it.
State what jenkins.sh does.
Print any build log that fails.

This makes it a heck of a lot easier to debug failures that happen only on Jenkins.
Destroy the vagrant box at the end of each Jenkins run.

This is what the vagrant Jenkins plugin does. I suspect state is leaking from run to run without this.
Upgrade to LLVM 3.2.

Use the binary tarball from llvm.org, since there are no good Ubuntu packages. This will let us merge #61.
Merge pull request #56 from cleeland/plugin_environment_variables

Export compiler command lines in DXR-specific env variables.
Merge support for controlling the order in which trees are built.
Make jenkins do "make clean" so old outputs can't confuse things.
Upgrade to LLVM 3.2. Closes #67.
Add "const" to the end of the fully qualified name of const methods. Closes #61.