[xmlsec] error message output

[Matthias Jung]

Aleksey, Igor

glad about the callback functionality to output error messages, I have 
written a new "xmlSecErrorsCallback" function.
This new function is very similar to the default 
("xmlSecErrorsDefaultCallback") function but uses my own logging mechanism.

Unfortunately I couldn't access the "xmlSecStrReasons" array because 
this variable was not exported and not accessable from xmlsec.dll (as 
you see it is Windows OS)
To help myself I have exported this array on my own.

In "errors.h" I have added the new declaration:
    XMLSEC_EXPORT_VAR ERR_STRING_DATA xmlSecStrReasons[];

Now this variable is accessable by my own callback function (defined in 
my application) and error message output to a logging file is fine.

In my eys this is a clever extension that should be adopted to the 
xmlsec project.
Is there a special reason why this was not done by now???

    Cheers Matthias