[ruby-core:78794] [Ruby trunk Bug#13060] pkg_config breaks when adding '-Wconversion' to $CFLAGS

Issue #13060 has been reported by Terence Lee.

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Bug #13060: pkg_config breaks when adding '-Wconversion' to $CFLAGS
https://bugs.ruby-lang.org/issues/13060

* Author: Terence Lee
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 2.4
* ruby -v: ruby 2.4.0rc1 (2016-12-12 trunk 57064) [x86_64-linux]
* Backport: 
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When compiling the nokogiri gem, you can opt to use system libraries which uses `pkg_config`. When I add '-Wconversion' to $CFLAGS, it causes 'pkg_config('libxml-2.0')' to return nil. This is not the case in Ruby 2.3.3.

This will affect anyone using nokogiri with Ruby 2.4.0 on Heroku.

test script:
~~~ ruby
require 'mkmf'

puts RUBY_DESCRIPTION
puts "pkg_config('libxml-2.0'): #{pkg_config('libxml-2.0').inspect}"
puts "Adding -Wconversion to $CFLAGS"
$CFLAGS << " -Wconversion"
puts "pkg_config('libxml-2.0'): #{pkg_config('libxml-2.0').inspect}"
~~~

ruby 2.3.3 output:
~~~
ruby 2.3.3p222 (2016-11-21 revision 56859) [x86_64-linux]
pkg_config('libxml-2.0'): ["", "", "-lxml2"]
Adding -Wconversion to $CFLAGS
pkg_config('libxml-2.0'): ["", "", "-lxml2"]
~~~

ruby 2.4.0-rc1 output:
~~~
ruby 2.4.0rc1 (2016-12-12 trunk 57064) [x86_64-linux]
pkg_config('libxml-2.0'): ["", "", "-lxml2"]
Adding -Wconversion to $CFLAGS
pkg_config('libxml-2.0'): nil
~~~



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