Add libxml2 include path for MSVC builds
Add libxml2 include path for MSVC builds

On Unix this path is detected via the use of xml2-config, but that's not
available on Windows. This means that users building with libxml2 will
no longer need to move things around from the standard libxml2
installation for MSVC builds.

Backpatch to all live branches.

lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb