pgsql: Check to see whether libxml2 handles error context the way we ex

Check to see whether libxml2 handles error context the way we expect.

It turns out to be possible to link against a libxml2.so that does this
differently than the version we configured and built against, so we need
a runtime check to avoid bizarre behavior.  Per report from Bernd Helmle.
Patch by Florian Pflug.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/c1420fcf7d9e899b3378e25f5e21776f6461b4a4

Modified Files
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src/backend/utils/adt/xml.c |   38 ++++++++++++++++++++++++++++++++++++--
1 files changed, 36 insertions(+), 2 deletions(-)