ENH: Generate native Xcode 3.0 and 3.1 projects
CMake previously generated Xcode project files labeled as 2.4-compatible by recent versions of Xcode (3.0 and 3.1). It is better to generate native Xcode 3.0 and 3.1 projects. In particular, this can improve build times by using the "Build independent targets in parallel" feature. Patch from Doug Gregor. See issue #9216.