Delete jsstddef.h, since its only remaining purpose is to make certain
Win16-specific definitions; we don't support Win16 any more. In
particular, we can just subtract pointers now, so the PTRDIFF macro is
unnecessary noise.
Most places get stddef.h via jstypes.h or some other header, so we can
just delete #inclusions of jstddef.h. The exception is jskwgen.h, so
there we explicitly include <stddef.h> instead.
We'd like the SpiderMonkey configure script to generate a header file
named js-config.h to hold #definitions chosen at configure time that
affect the SpiderMonkey API, like JS_THREADSAFE. However, that name
is very similar to that of an existing header file, jsconfig.h. This
patch renames the existing header file, and updates all references to
it.
