[xml] buildDocBookCatalog script enhancement

["William M. Brack" <wbrack mmm com hk>]

In response to a bugzilla request for enhancement, I have added some
features to the buildDocBookCatalog script (available from
http://xmlsoft.org/buildDocBookCatalog).  The following is a brief
description of the changes.

1) With no parameters, the script will now take account of the
environment variable XML_CATALOG_FILES, i.e. if the variable is set 
the resulting files will be put into that directory providing that
the --prefix parameter is not set.

2) The user may specify a parameter --prefix=DIR, in which case the
default directory will be replaced by the directory "DIR".  In this
instance the files produced will be "catalog" and "docbook".

3) If the directory to be used for the files is not writable, or the
user does not have write access to the files, the target directory
will be changed to the user's home directory.  In this case, the
generated files will be named "xmlcatalog" and "dbkxmlcatalog" (the
previous script had a similar behaviour).

4) The user may specify a parameter --show which, if present, will
cause the script to display the full pathnames where the files will
be put, but not actually do the file generation.

5) By default, the script generates docbook files for version 4.1.2.
 The user may specify a parameter, --version=x.y.z, in which case
the script will (attempt) to generate the files for the specified
version.

If any problems are encountered with this enhancement, please report
them either to this list, or as a bug via bugzilla.

Bill