libxml2 is dropping xml2-config

Debian reports that libxml2 is dropping the xml2-config binary:

Date: Mon, 20 Jan 2020 20:42:47 +0100
From: Mattia Rizzolo <mattia(at)debian(dot)org>
Reply-To: Mattia Rizzolo <mattia(at)debian(dot)org>, 949428(at)bugs(dot)debian(dot)org
Subject: Bug#949428: postgresql-12: FTBFS with libxml2 2.9.10 (uses xml2-config)

Source: postgresql-12
Version: 12.1-2
Severity: important
Tags: ftbfs
User: libxml2(at)packages(dot)debian(dot)org
Usertags: ftbfs-2.9.10 xml2-config

Dear maintainer,

your package is using `xml2-config` to detect and use libxml2. I'm
removing that script, so please update your build system to use
pkg-config instead.

The libxml2 package in experimental already doesn't ship the xml2-config
script.

Attached is the full build log, hopefully relevant excerpt follows:

checking for xml2-config... no
...
configure: error: header file <libxml/parser.h> is required for XML support

[...]
----- End forwarded message -----

Luckily the ./configure script is compatible enough so that this hack
works: (tested on master)

./configure --with-libxml XML2_CONFIG='pkg-config libxml-2.0'
[...]
checking for XML2_CONFIG... pkg-config libxml-2.0
[...]
checking for xmlSaveToBuffer in -lxml2... yes
[...]
checking libxml/parser.h usability... yes
checking libxml/parser.h presence... yes
checking for libxml/parser.h... yes

We should teach configure.in to recognize that natively as well, I
guess.

Christoph