xmlsec-c port for Xerces 3

[Scott Cantor <ca...@osu.edu>]

In case anybody (else) cares, I'm working on getting the xmlsec trunk to
build against Xerces 3.0. Most of the work was done earlier by Berin, but
after he stopped working on it, the Xerces API moved again, including a
bunch of changes to parameter types for 64-bit safety.

I have the trunk building on Windows and will be testing the Unix port
shortly, followed by a 1.5.0 release early next year. I have a couple of
small things I want to add to the API after it's working on Xerces 3.

It should be source-compatible with 1.4 when using Xerces 2.x, but may
require some source changes to use with Xerces 3.x because of the 64-bit
changes there (this is in addition to whatever you might have to do just to
port your code to Xerces 3 itself).

As an aid to portability across the Xerces versions, some of the xmlsec APIs
now rely on a typedef, xsecsize_t, which expands to unsigned int for Xerces
2.x and to XMLSize_t for Xerces 3.x. Code can be modified to use that
typedef to make it possible to build against either Xerces version, at least
to some degree. I don't like adding yet another alias for what should just
be size_t, but this is fallout from the chain of API mistakes, and is the
only good way to support either Xerces version.

-- Scott