Second attempt at committing mods for OS X 10.8 clang... as before:

Commit of changes to get things running with Xcode 5's clang compiler
on OS X 10.8. In particular, added a conditional macro to *not* call
realloc() in read_CPL_privatecode() and read_CPLcode_into_property ()
when using clang, since clang's implementation moves memory even when
downsizing. Other critical fixes included correcting the size of the
sortdatat to be memmoved in copy_elem(), and fixing a bug in
kill_edge_make_ring() where del_le() was called on a boundary that
wasn't initialized correctly.  Also added an Xcode project file, and
removed extern.h in the top level directory.

Also, some minor Makefile.common changes to prevent creation of empty
extern.h file in top directory.