added CACAO to IcedTea's build system

[Christian Thalinger]

Hi!

As Andrew Haley said today on IRC about building IcedTea with CACAO as
VM:

<aph> twisti just says "it's easy" and you then spend 3 days with him on
IRC

I finally integrated building CACAO to the IcedTea build system.  So,
when you're now using --with-cacao, it downloads the latest release
(0.99.1), builds it and the resulting libjvm.so is then used via
ALT_HOTSPOT_IMPORT_PATH.

I have also added two new options:

--with-cacao-home
--with-cacao-src-zip

These two are like the openjdk ones: --with-cacao-home uses an
externally built CACAO and --with-cacao-src-zip uses an already
downloaded tarball.

Have fun!

- twisti