Hi Guix,

this patch adds ant-minimal, a minimal configuration of Apache Ant.  Ant
can be configured with lots of optional libraries, but since most of
these libraries are built with Ant, I think it makes sense to provide a
minimal version.

A wart is that Ant (even in the minimal configuration) depends on
hamcrest-core, which can only be built with Ant.  The good news is that
it appears that hamcrest-core is only used for running the tests after
Ant is built.

I used the gnu-build-system instead of the trivial-build-system, because
using the trivial-build-system required me to write a lot more code in
order to unpack the tarball, patch shebangs, add tools to the PATH,
etc.  Using the gnu-build-system I only had to remove a few phases to
make it work.  Maybe we need a somewhat more powerful version of the
trivial-build-system.

~~ Ricardo