Reviewed by Sam Weinig and Tim Hatcher.

Move base setting for HEADER_SEARCH_PATHS into Base.xcconfig, and extend
it in JavaScriptCore.xcconfig.  This removes the need to override it on a
per-target basis inside the .xcodeproj file.

* Configurations/Base.xcconfig:
* Configurations/JavaScriptCore.xcconfig:
* JavaScriptCore.xcodeproj/project.pbxproj:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@27845 268f45cc-cd09-0410-ab3c-d52691b4dbfc