On Darwin, ignore anything but system's ranlib. Fixes #1812

[gpakosz]

This PR makes it so the call to libxml2's configure script is done with passing RANLIB=/usr/bin/ranlib when running on Darwin.

This makes building native extensions more robust to the target system in the following situation:

• the user is using macOS and
• the user has GNU binutils in her PATH
• or somewhat, GNU binutils are linked in /usr/local/bin

In particular, having libxml2 build call GNU's ranlib instead of Darwin's ranlib fails building native extensions with very cryptic error messages. The root cause is far from being obvious and one can waste many hours on it.

As such, I suggest trying to spare users and maintainers' time (less issues opened) by helping libxml2's build system a bit.

[flavorjones]

This is related to #1812

[flavorjones]

/cc @zenspider Can you please take a look at this.

[nurse]

@zenspider @flavorjones I hit this many times for each my environments...
When will you merge and release new nokogiri gem?

[flavorjones]

@nurse Can you confirm that this fix addresses your issue? I have no easy way to test.

[gpakosz]

@nurse Now I'm curious. Do you have GNU's ranlib in your PATH in each of your environments? Why is it so?

[nurse]

@flavorjones It also needs AR=/usr/bin/ar and libxslt also needs the hack. With such change, I confirmed it works on my environment.

@gpakosz I want to have addr2line and GNU's objdump.
Well but there's few people who want to do that..

[flavorjones]

Will take a look in the morning. The failing tests in CI are unrelated.

[flavorjones]

Merged into master.

[nurse]

Thank you for releasing 1.9.0.rc1 and I successfully installed it!

%  ruby -v
ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-darwin16]
%  gem install nokogiri --pre
Fetching: mini_portile2-2.4.0.gem (100%)
Successfully installed mini_portile2-2.4.0
Fetching: nokogiri-1.9.0.rc1.gem (100%)
Building native extensions. This could take a while...
Successfully installed nokogiri-1.9.0.rc1
Parsing documentation for mini_portile2-2.4.0
Installing ri documentation for mini_portile2-2.4.0
Parsing documentation for nokogiri-1.9.0.rc1
Installing ri documentation for nokogiri-1.9.0.rc1
Done installing documentation for mini_portile2, nokogiri after 13 seconds
2 gems installed
%  /usr/local/bin/ranlib --version
GNU ranlib (GNU Binutils) 2.31.1
Copyright (C) 2018 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or (at your option) any later version.
This program has absolutely no warranty.