[xml/sgml-pkgs] Bug#1092484: configure libxml2 without ICU support

[Matthias Klose]

Package: src:libxml2
Version: 2.12.7+dfsg+really2.9.14-0.2
Severity: important
User: debian-devel at lists.debian.org
Usertags: icu76

libxml's README.md states:

- [ICU](https://icu.unicode.org/), a Unicode library. Mainly
   useful as an alternative to iconv on Windows. Unnecessary
   on most other systems.

ICU 76.1 requires to be built with -std=c++17 or -std=gnu++17 or higher. 
However including the ICU headers in the libxml2 headers, breaks builds 
with older C++ standards, most likely leading to some unrelated build 
failures for packages that don't rely on ICU, but are using libxml2.

Is there a rationale for building with ICU?  I see that Fedora/RedHat 
packages also don't build with ICU.