gccxml path asked during C++/Python construction

The interface construction crashes when the gccxml path is not defined in the file interface.py.
The error message is:
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poulain@cumin> python interface.py
Traceback (most recent call last):
  File "interface.py", line 14, in <module>
    , define_symbols=[] )
  File "/home/cumin/poulain/soft/lib/python2.5/site-packages/pyplusplus/module_builder/builder.py", line 92, in __init__
    , indexing_suite_version)
  File "/home/cumin/poulain/soft/lib/python2.5/site-packages/pyplusplus/module_builder/builder.py", line 146, in __parse_declarations
    decls = reader.read_files( files, compilation_mode )
  File "/home/cumin/poulain/soft/lib/python2.5/site-packages/pygccxml/parser/project_reader.py", line 225, in read_files
    return self.__parse_file_by_file(files)
  File "/home/cumin/poulain/soft/lib/python2.5/site-packages/pygccxml/parser/project_reader.py", line 246, in __parse_file_by_file
    , self.__decl_factory )
  File "/home/cumin/poulain/soft/lib/python2.5/site-packages/pygccxml/parser/source_reader.py", line 87, in __init__
    self.__config.raise_on_wrong_settings()
  File "/home/cumin/poulain/soft/lib/python2.5/site-packages/pygccxml/parser/config.py", line 189, in raise_on_wrong_settings
    raise RuntimeError( msg )
RuntimeError: gccxml_path(" ") should exists or to be a valid file name.
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OS: Fedora Core 10
boost 1.34, boost devel 1.34, python-devel 2.5.2, scons 1.0.0 all from official Fedora repositories
gccxml 0.9.0 compiled from svn repository (download and compiled 09/03/2010)
pygccxml and py++ 1.0.0 from original website
PATH contains gccxml path
PYTHONPATH contains py++ and pygccxml paths

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Defining gccxml path in the file interface.py (line 13) works:

, gccxml_path=r"/home/cumin/poulain/soft/gccxml/bin"

instead of

, gccxml_path=r""

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Question:
1/ how py++ and pygccxml were doing before to find gccxml path ?
2/ how to write gccxml path in interface.py in a dynamic/automatic way ?