[integrator] Add build-specific xcconfig to builds

Also fixes some typos from previous commit. Different xcconfigs are now
actually generated, correctly assigned, and it all builds! Doesn't pass
tests and all xcconfigs are still identical, but still: getting there!

lib/cocoapods/installer/target_installer/pod_target_installer.rb

@@ -84,7 +84,7 @@ module Pod
       # @return [void]
       #
       def create_xcconfig_file
-        path = library.xcconfig_path
+        path = library.xcconfig_path nil
         public_gen = Generator::XCConfig::PublicPodXCConfig.new(library)
         UI.message "- Generating public xcconfig file at #{UI.path(path)}" do
           public_gen.save_as(path)

lib/cocoapods/installer/user_project_integrator/target_integrator.rb

@@ -108,11 +108,12 @@ module Pod
         # @return [void]
         #
         def add_xcconfig_base_configuration
-          xcconfig = user_project.files.select { |f| f.path == target.xcconfig_relative_path }.first ||
-                     user_project.new_file(target.xcconfig_relative_path)
           native_targets.each do |native_target|
-            check_overridden_build_settings(target.xcconfig, native_target)
             native_target.build_configurations.each do |config|
+              relpath = target.xcconfig_relative_path config.name
+              xcconfig = user_project.files.select { |f| f.path == relpath }.first || user_project.new_file(relpath)
+
+              check_overridden_build_settings(target.xcconfigs[config.name], native_target)
               config.base_configuration_reference = xcconfig
             end
           end

lib/cocoapods/target.rb

@@ -77,10 +77,16 @@ module Pod
       @sandbox.library_support_files_dir(name)
     end
 
+    # @param  [String] variant (optional) If you have multiple variants of xcconfigs for this target
+    # =>                       (e g per build configuration), provide the variant type here.
     # @return [Pathname] the absolute path of the xcconfig file.
     #
     def xcconfig_path variant
-      support_files_root + "#{label}-#{variant}.xcconfig"
+      if variant then
+        support_files_root + "#{label}.#{variant}.xcconfig"
+      else
+        support_files_root + "#{label}.xcconfig"
+      end
     end
 
     # @return [Pathname] the absolute path of the private xcconfig file.

lib/cocoapods/target/aggregate_target.rb

@@ -97,7 +97,7 @@ module Pod
     #         the user project.
     #
     def xcconfig_relative_path config_name
-      relative_to_srcroot(xcconfig_path, config_name).to_s
+      relative_to_srcroot(xcconfig_path config_name).to_s
     end
 
     # @return [String] The path of the copy resources script relative to the