clang has __builtin_add_overflow and __builtin_sub_overflow, like GCC.
No code changes are needed because:
* The definition of _GL_HAS_BUILTIN_ADD_OVERFLOW in lib/intprops.h works
fine for clang.
* The function size_add_wrapv in lib/glob.c produces identical code with
the __builtin_add_overflow as without. Apparently clang recognizes the
addition-and-comparison pattern.Bruno
