bug#34849: Compilation issues with g++ on some files

[Eli Zaretskii <eliz@gnu.org>]

> From: Richard Stallman <rms@gnu.org>
> Date: Mon, 18 Mar 2019 22:24:18 -0400
> Cc: eggert@cs.ucla.edu, 34849@debbugs.gnu.org
> 
> 1. I don't want C++ code in the Emacs distribution.  This isn't the sort
> of crucial thing that would override that general policy.

I believe the intent was to make the existing code be compilable with
a C++ compiler without introducing any C++ code per se.

My POV on this is that we should make Emacs buildable with as many
modern GUI toolkits as practically possible/reasonable, because it is
not clear which one(s) of them will remain workable and maintained in
the long run.  And since many/most of the toolkits and packages we'd
like to use or be compatible with are written in C++ (HarfBuzz is a
good recent example), we should try making our sources be more
friendly to C++ compilers, as much as possible, because not every such
package has a C glue, like HarfBuzz does.

Therefore, if integration with Qt is reasonably possible, we should
not reject it outright.  I understand the concerns regarding the
license, but we will need to revisit that when GPL's version is
advanced, and not sooner.  Disabling support for a package that no
longer satisfies our requirements is relatively easy.