pod_target_xcconfig ends up applied to aggregate target

[pixelglow]

In the Podspec Syntax Reference, the pod_target_xcconfig setting is documented as applying only to the private pod target xcconfig file, but ends up applying to the aggregate/project target as well.

Steps to reproduce:

• Create a Xcode iOS project (6.3 or later).
• Use the following podfile in the project directory:

platform :ios, '8.0'
pod 'zipzap'

• Run pod install from that directory.
• Open the new xcworkspace file.
• Note that the created Pods.debug.xcconfig and Pods.release.xcconfig files contain CLANG_CXX_LANGUAGE_STANDARD=c++11, OTHER_CPLUSPLUSFLAGS=-fno-exceptions etc. and the project uses these configurations.
• Note that in the current zipzap podspec, these flags are found in the pod_target_xcconfig setting and nowhere else.

Reference: pixelglow/ZipZap#105

Versions: Xcode 6.3.2, CocoaPods 0.38.2, ZipZap 8.0.4

[segiddins]

@mrackwitz this seems to be caused by https://github.com/CocoaPods/CocoaPods/blob/master/lib/cocoapods/generator/xcconfig/aggregate_xcconfig.rb#L143, which would mean that any non-framework targets don't actually see separation of xcconfigs.