-Wuninitialized is not supported without -O

If you get the error above, it is telling you that you have uninitialized variables (-Wuninitialized) and apparently gcc can only figure things out if optimizations is turned on (that's the -O).  For debug builds optimizations are usually turned off.  So in Xcode 4, you'll need to go to your build settings, look for "Uninitialized Automatic Variables" and turn it off for Debug.

Source: http://forum.soft32.com/mac/Wuninitialized-supported-ftopict45197.html