[build] Merging changes from fed.mrh branch

[Mark Hindess <ma...@googlemail.com>]

Unless anyone objects, I plan to merge some changes from the mrh branch
tomorrow - everything except the drlvm native restructuring.

The main changes are:

* common_resources will be required to build classlib - as I'm removing
  the duplicate make/*.xml files from classlib in favour of the ones in
  common_resources

* the target directory for the build is now specified using hy.target
  rather than hy.hdk ... the default value of hy.target is 'target' and
  the default value for hy.hdk is now ${hy.target}/hdk

* all generated files[0] are created under ${hy.target}/build

* all downloaded dependencies are placed in ${hy.depends} which defaults
  to ${hy.target}/depends - so there is only one copy of any dependency
  jar now.

* you can specify the ant option:

    -Dhy.depends=$HOME/harmony-depends

  to have only one copy of all dependencies for several workspaces.  You
  could use ANT_ARGS environment variable to apply this to all ant runs.
  
* classlib and jdktools natives have dependency support at build time
  on platforms using gcc (with dependencies generated on the fly with
  -MD compiler option).

Regards,
-Mark.

[0] Not quite all yet actually but most.