Added an option for configure '--with-piddir=>directory>'.

When not specified it is the same as 'WORKDIR'.

This came up when building the debian package. We need to change the location of the pid files.  For testing we can override it with an environment variable 'CFENGINE_TEST_OVERRIDE_PIDDIR' and also defined 'sys.piddir', eg:

body common control
{
bundlesequence => { test };
}

bundle agent test
{
reports:
        any::
            "$(sys.workdir)";
            "$(sys.piddir)";
}