Create WebKit availability macros that key off the Mac OS X version being targeted to
determine the WebKit version being targeted.  Applications can define
WEBKIT_VERSION_MIN_REQUIRED before including WebKit headers in order to target a specific
version of WebKit.

The availability header is being added to JavaScriptCore rather than WebKit as JavaScriptCore
is the lowest-level portion of the public WebKit API.

Reviewed by Tim Hatcher.

* API/WebKitAvailability.h: Added.
* JavaScriptCore.xcodeproj/project.pbxproj:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@33053 268f45cc-cd09-0410-ab3c-d52691b4dbfc