In the advanced ant option, it's possible to use a different file but it's not
possible to specify the directory the build should be run from.

Let me use an example to try to explain the problem. In pmd-4.2, ant must be
started as:
ant -f bin/build.xml <target>

If I enter "bin/build.xml" in the "Build File" text field, ant is started from
the bin directory. It's also the case when using an absolute file name for
build.xml.

An extra option to specify the directory from where to run ant would help solve
the problem.

PS: I know this is not a standard way to use ant and it's been fixed in the pmd
trunk, build.xml is now at the top level...