* __scm.h: move libguile/scmconfig.h include up to the top, so

* __scm.h: move libguile/scmconfig.h include up to the top, so
we're sure to pick up any critical defines like _GNU_SOURCE early.
#include <limits.h> removed in favor of scmconfig.h inclusion when
appropriate.  STDC_HEADERS based inclusion of stdlib.h,
sys/types.h, stddef.h, and sys/stdtypes.h removed in favor of
scmconfig.h inclusion when appropriate.  Various Win32 related
definitions removed in favor of scmconfig.h inclusion when
appropriate.
(HAVE_UINTPTR_T): definition removed (see NEWS).
(SIZEOF_PTRDIFF_T): definition removed (see NEWS).
(HAVE_LONG_LONGS): definition removed (see NEWS).
(HAVE_LONG_LONG): definition removed (see NEWS).
(HAVE_PTRDIFF_T): definition removed (see NEWS).