bug#28571: 25.3; Compile error on macOS Sierra

[YAMAMOTO Mitsuharu]

>>>>> On Sat, 30 Sep 2017 19:17:33 +0000, Philipp Stephani 
>>>>> <p.stephani2@gmail.com> said:

>> Probably You need to execute "$ xcode-select --install" to install
>> Command Line Tools for Xcode.  You can actually go without it by
>> tweaking some environment variables and/or compiler arguments, but
>> I wouldn't recommend that unless you have a shortage of disk space
>> and you really understand what you are doing.
>> 
>> 
> OK, the warning is indeed gone after that, thanks.  Should we put
> something in configure.ac that checks for this? It's a bit subtle
> because compilation almost completely works without it.

Installation of the Command Line Tools is not a strict requirement.
For example, the Nix package manager does not require Xcode but use
its own SDK, IIUC.  On the other hand, MacPorts and Homebrew
explicitly require users to install the Command Line Tools:

  https://www.macports.org/install.php
  https://docs.brew.sh/Installation.html

I guess nowadays most users use some package manager.  If they are
following its installation instructions correctly, they are in a
correct configuration.  Otherwise, they will face another problem
sooner or later anyway.

                             YAMAMOTO Mitsuharu
                        mituharu@math.s.chiba-u.ac.jp