Reviewed by Geoff Garen.

        - fixed <rdar://problem/5806316> JavaScriptCore should not force building with gcc 4.0
        - use gcc 4.2 when building with Xcode 3.1 or newer on Leopard, even though this is not the default

        This time there is no performance regression; we can avoid having
        to use the fastcall calling convention for CTI functions by using
        varargs to prevent the compiler from moving things around on the
        stack.
        
        * Configurations/DebugRelease.xcconfig:
        * JavaScriptCore.xcodeproj/project.pbxproj:
        * VM/CTI.cpp:
        * VM/Machine.h:
        * wtf/Platform.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@37647 268f45cc-cd09-0410-ab3c-d52691b4dbfc