need more control over error/warning message printing

https://bugs.documentfoundation.org/show_bug.cgi?id=92692

several LibreOffice users have complained about confusing warning messages on stderr in release builds, apparently printed by liblangtag.

W: Unknown node under /registry/extlang: deprecated
W: Unknown node under /registry/grandfathered: comments
W: Unknown node under /registry/grandfathered: comments

in general we don't want to print anything in release builds other than fatal error messages on crashes, because users tend to be easily scared and will think whatever message is printed must have something to do with whatever totally unrelated bug they see.

libxml2 has some hooks to set a custom error handling functions, which we already use to avoid printing the error messages (except in debug builds):

http://xmlsoft.org/html/libxml-xmlerror.html#xmlSetGenericErrorFunc

there is apparently something similar in lt-messages.h lt_message_set_handler but it's not a public header, or at least Fedora 22 liblangtag-devel does not contain this file.

would it be possible to have something like this as a public API?

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