The former is deprecated with C++14. The standard says both are the same:

The contents and meaning of the header<cstddef>are the same as the C
standard library header<stddef.h>,except that it does not declare the type
wchar_t, that it also declares the type byte and its associated
operations (21.2.5), and as noted in 21.2.3 and 21.2.4.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Reduce header dependencies.

Fixes integer truncation bugs (closes #359).

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Basic definition to solve http://bugs.musicpd.org/view.php?id=3147 and
http://bugs.musicpd.org/view.php?id=3255