stdint: be more consistent with glibc, SunOS libc

stdint: be more consistent with glibc, SunOS libc

* lib/stdint.in.h (gl_int_fast8_t, gl_uint_fast8_t)
(gl_int_fast16_t, gl_uint_fast16_t)
(gl_int_fast32_t, gl_uint_fast32_t)
(INT_FAST8_MIN, INT_FAST8_MAX, UINT_FAST8_MAX)
(INT_FAST16_MIN, INT_FAST16_MAX, UINT_FAST16_MAX)
(INT_FAST32_MIN, INT_FAST32_MAX, UINT_FAST32_MAX):
Be consistent with glibc by default, and with SunOS 5.10 and later
if __sun is defined.  This lessens the likelihood of clashes if
code compiled for older hosts is combined with code compiled for
newer ones.  Problem reported by Niels Möller in
<http://lists.gnu.org/archive/html/bug-gnulib/2012-05/msg00074.html>.