Build fix after r160557.

Source/JavaScriptCore:

r160557 added the first generated header to JavaScriptCore that needs to be installed in to
the framework wrapper. Sadly JavaScriptCore's Derived Sources target was not set to generate
headers when invoked as part of the installhdrs action. This resulted in the build failing
due to Xcode being unable to find the header file to install. The fix for this is to configure
the Derived Sources target to use JavaScriptCore.xcconfig, which sets INSTALLHDRS_SCRIPT_PHASE
to YES and allows Xcode to generate derived sources during the installhdrs action.

Enabling INSTALLHDRS_SCRIPT_PHASE required tweaking the Generate Derived Sources script build
phase to skip running code related to offlineasm that depends on JSCLLIntOffsetExtractor
having been compiled, which isn't the case at installhdrs time.

* JavaScriptCore.xcodeproj/project.pbxproj:

Source/WebCore:

* Configurations/WebCore.xcconfig: Find JavaScriptCore.framework below SDKROOT so that we'll pick
up the built version in production builds rather than the system version.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@160594 268f45cc-cd09-0410-ab3c-d52691b4dbfc