Use the Windows gemspec in the Gemfile on Windows.

Gemfile

@@ -1,6 +1,10 @@
 source "https://rubygems.org"
 
-gemspec
+if !!File::ALT_SEPARATOR
+  gemspec :name => 'rest-client.windows'
+else
+  gemspec
+end
 
 group :test do
   gem 'rake'

rest-client.windows.gemspec

@@ -8,7 +8,7 @@
 
 s = eval(File.read(File.join(File.dirname(__FILE__), 'rest-client.gemspec')))
 
-platform = ENV['BUILD_PLATFORM']
+platform = ENV['BUILD_PLATFORM'] || RUBY_PLATFORM
 
 case platform
 when /(mingw32|mswin32)/