enable icu support by default in libxml2

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

markd@ packaged qt5-qtwebengine for pkgsrc-wip. It needs libxml2 built
with icu support enabled (which is an option for the libxm2 package,
currently default-off).

ISTR that we had other programs in the past that wanted ICU support in
libxml2 (though I forgot the actual cases).

Is it time to switch the default to turn on icu by default in libxml2?

(If we don't do that, qt5-qtwebengine will need to build against its
included libxml2, and we'll have to patch libxml2 problems/security
issues in that code base as well.)

Cheers,
 Thomas