Add tracing infrastructure.

Adds the tracing instruction:
     TRACE(t, l, __ format arg1 arg2 ... argn)
where t is a tracer (that can be enabled on the command line) and
l is a level. Any TRACE whose level is below the one specified on
the command line for the tracer is printed.

The project is built without -trace, the TRACE() command is a noop.

git-svn-id: svn+ssh://idea.nguyen.vg/svn/sxsi/trunk/xpathcomp@1194 3cdefd35-fc62-479d-8e8d-bae585ffb9ca