Platform-neutral build no longer has native extensions.

Author: jdantonio

concurrent-ruby.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |s|
   if defined?(JRUBY_VERSION)
     s.files += Dir['lib/concurrent_ruby_ext.jar']
     s.platform = 'java'
-  else
+  elsif ! ENV['BUILD_PURE_RUBY']
     s.extensions = 'ext/concurrent_ruby_ext/extconf.rb'
     s.files += Dir['ext/**/*.{h,c,cpp}']
   end

lib/concurrent/atomic.rb

@@ -11,7 +11,7 @@
 
   require "concurrent/atomic_reference/#{ruby_engine}"
 rescue LoadError
-  warn "Unsupported Ruby engine `#{RUBY_ENGINE}', using less-efficient Atomic impl"
+  warn 'Compiled extensions not installed, pure Ruby Atomic will be used.'
 end
 
 if defined? Concurrent::JavaAtomic