ant-build-system: Allow specifying source directory.

* guix/build-system/ant.scm (ant-build),
guix/build/ant-build-system.scm (default-build.xml): Add parameter
source-dir.
* guix/build/ant-build-system.scm (configure): Pass source-dir on to
default-build.xml.
* doc/guix.texi (Build Systems): Document it.

Co-authored-by: Ricardo Wurmus <rekado@elephly.net>