Libxml2 load error on Windows

Hi,

PostgreSQL 9.2 Windows build is having trouble with the XML support:
http://postgresql.1045698.n5.nabble.com/9-2-beta1-libxml2-can-t-be-loaded-on-Windows-td5710672.html

postgres=# SELECT xml '<foo>bar</foo>';
ERROR:  could not set up XML error handler
HINT: This probably indicates that the version of libxml2 being used is not
compatible with the libxml2 header files that PostgreSQL was built with.

HAVE_XMLSTRUCTUREDERRORCONTEXT should be defined if libxml2 has
xmlStructuredErrorContext
(introduced after 2.7.3). It is being set for Linux in pg_config.h:

/* Define to 1 if your libxml has xmlStructuredErrorContext. */
#define HAVE_XMLSTRUCTUREDERRORCONTEXT 1

For Windows build, we need to set it in "src/tools/msvc/Solution.pm".

I guess there are two approaches to get the libxml2 version:

1) Parse "LIBXML_INST/include/libxml/xmlversion.h" to get the libxml2 version
number, this header file is included in libxml2.

2) We may also get the version by running a command line utility i.e
"LIBXML_INST/bin/xmllint
--version". The xmllint program parses one or more XML files and it is also
included in libxml2.

I believe it is safe to assume that libxml2 include folder would contain
xmlversion.h file containing the required version number. It might be a
touch slower to parse the file line by line, but similar functionality has
been used elsewhere as well. We obviously rely on xml include files (with
xml enabled), so there is definitely a tighter dependency on include files
than xmllint executable as it is not mandatory for build process.

Therefore, I used first approach to get libxml2 version number and include
the "#define HAVE_XMLSTRUCTUREDERRORCONTEXT" in pg_config.h if libxml2 version
is greater than 20703 (which effectively is 2.7.3). Please find attached
patch "libxml2_win_v1.patch".

Regards,
Talha Bin Rizwan