Modify add_xcode_base_configuration to respect existing base

configurations:
 - Do not change the base configuration when it is already set
 - Notify the user when there is an existing base configuration

Change string quoting from double to single quotes.

Split lines longer than 80 characters into multiple lines.

Quote initial string fragment with single quotes rather than double quotes.

CHANGELOG.md

@@ -16,6 +16,10 @@ To install or update CocoaPods see this [guide](http://docs.cocoapods.org/guides
   [Samuel Giddins](https://github.com/segiddins)
   [#2577](https://github.com/CocoaPods/CocoaPods/issues/2577)
 
+* Integration with user project will no longer replace an existing
+  base build configuration.  
+  [Robert Jones](https://github.com/redshirtrob)
+
 ##### Bug Fixes
 
 * Improved sanitizing of configuration names to avoid generating invalid

lib/cocoapods/installer/user_project_integrator/target_integrator/xcconfig_integrator.rb

@@ -19,7 +19,7 @@ module Pod
             targets.each do |target|
               target.build_configurations.each do |config|
                 update_to_cocoapods_0_34(pod_bundle, targets)
-                set_target_xcconfig(pod_bundle, config)
+                set_target_xcconfig(pod_bundle, target, config)
               end
             end
           end
@@ -35,7 +35,7 @@ module Pod
           # @param  [Target::AggregateTarget] pod_bundle
           #         The Pods bundle.
           #
-          # @param  [XcodeProj::PBXNativeTarget] target
+          # @param  [Array<XcodeProj::PBXNativeTarget>] targets
           #         The native targets.
           #
           # @todo   This can be removed for CocoaPods 1.0
@@ -67,15 +67,25 @@ module Pod
           # @param  [Target::AggregateTarget] pod_bundle
           #         The Pods bundle.
           #
-          # @param  [[Xcodeproj::XCBuildConfiguration] config
+          # @param  [PBXNativeTarget] target
+          #         The native target.
+          #
+          # @param  [Xcodeproj::XCBuildConfiguration] config
           #         The build configuration.
           #
-          def self.set_target_xcconfig(pod_bundle, config)
+          def self.set_target_xcconfig(pod_bundle, target, config)
             path = pod_bundle.xcconfig_relative_path(config.name)
             group = config.project['Pods'] || config.project.new_group('Pods')
             file_ref = group.files.find { |f| f.path == path }
-            file_ref ||= group.new_file(path)
-            config.base_configuration_reference = file_ref
+            if config.base_configuration_reference != file_ref
+              UI.warn "CocoaPods did not set the base configuration of your " \
+              "project because one is already set. Please either set the " \
+              "base configurations of the target `#{target.name}` to " \
+              "`#{path}` or include the `#{path}` in your build configuration."
+            else
+              file_ref ||= group.new_file(path)
+              config.base_configuration_reference = file_ref
+            end
           end
 
           private

spec/unit/installer/user_project_integrator/target_integrator/xcconfig_integrator_spec.rb

@@ -48,5 +48,19 @@ module Pod
       config.base_configuration_reference.should.equal existing
     end
 
+    it 'does not set the Pods xcconfig as the base config if the base ' \
+       'config is already set' do
+      sample_config = @project.new_file('SampleConfig.xcconfig')
+      @target.build_configurations.each do |config|
+        config.base_configuration_reference = sample_config
+      end
+      XCConfigIntegrator.integrate(@pod_bundle, [@target])
+      @target.build_configurations.each do |config|
+        config.base_configuration_reference.should == sample_config
+      end
+
+      UI.warnings.should.match /not set.*base configuration/
+    end
+
   end
 end