Improve cross-compilation guesses for native Windows.

Improve cross-compilation guesses for native Windows.

* m4/btowc.m4 (gl_FUNC_BTOWC): Add cross-compilation guess for native
Windows.
* m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): Likewise.
* m4/calloc.m4 (_AC_FUNC_CALLOC_IF): Likewise.
* m4/canonicalize.m4 (gl_FUNC_REALPATH_WORKS): Likewise.
* m4/cbrtf.m4 (gl_FUNC_CBRTF_WORKS): Likewise.
* m4/cbrtl.m4 (gl_FUNC_CBRTL, gl_FUNC_CBRTL_WORKS): Likewise.
* m4/ceil.m4 (gl_FUNC_CEIL): Likewise.
* m4/ceilf.m4 (gl_FUNC_CEILF): Likewise.
* m4/ceill.m4 (gl_FUNC_CEILL): Likewise.
* m4/d-ino.m4 (gl_CHECK_TYPE_STRUCT_DIRENT_D_INO): Likewise.
* m4/dup.m4 (gl_FUNC_DUP): Likewise.
* m4/expl.m4 (gl_FUNC_EXPL): Likewise.
* m4/exp2.m4 (gl_FUNC_EXP2_WORKS): Likewise.
* m4/exp2l.m4 (gl_FUNC_EXP2L): Likewise.
* m4/expm1.m4 (gl_FUNC_EXPM1): Likewise.
* m4/expm1f.m4 (gl_FUNC_EXPM1F_WORKS): Likewise.
* m4/exponentl.m4 (gl_LONG_DOUBLE_EXPONENT_LOCATION): Likewise.
* m4/fabsl.m4 (gl_FUNC_FABSL_WORKS): Likewise.
* m4/fchdir.m4 (gl_FUNC_FCHDIR): Likewise.
* m4/fcntl-o.m4 (gl_FCNTL_O_FLAGS): Likewise.
* m4/float_h.m4 (gl_FLOAT_H): Likewise.
* m4/floor.m4 (gl_FUNC_FLOOR): Likewise.
* m4/floorf.m4 (gl_FUNC_FLOORF): Likewise.
* m4/fma.m4 (gl_FUNC_FMA_WORKS): Likewise.
* m4/fmaf.m4 (gl_FUNC_FMAF_WORKS): Likewise.
* m4/fmal.m4 (gl_FUNC_FMAL_WORKS): Likewise.
* m4/fmod.m4 (gl_FUNC_FMOD): Likewise.
* m4/fmodf.m4 (gl_FUNC_FMODF): Likewise.
* m4/fmodl.m4 (gl_FUNC_FMODL): Likewise.
* m4/frexp.m4 (gl_FUNC_FREXP_WORKS): Likewise.
* m4/frexpf.m4 (gl_FUNC_FREXPF_WORKS): Likewise.
* m4/ftello.m4 (gl_FUNC_FTELLO): Likewise.
* m4/gettimeofday.m4 (gl_FUNC_GETTIMEOFDAY_CLOBBER): Likewise.
* m4/hypot.m4 (gl_FUNC_HYPOT): Likewise.
* m4/hypotf.m4 (gl_FUNC_HYPOTF, gl_FUNC_HYPOTF_WORKS): Likewise.
* m4/hypotl.m4 (gl_FUNC_HYPOTL, gl_FUNC_HYPOTL_WORKS): Likewise.
* m4/ilogb.m4 (gl_FUNC_ILOGB_WORKS): Likewise.
* m4/ilogbf.m4 (gl_FUNC_ILOGBF_WORKS): Likewise.
* m4/isfinite.m4 (gl_ISFINITEL_WORKS): Likewise.
* m4/isinf.m4 (gl_ISINFL_WORKS): Likewise.
* m4/isnanf.m4 (gl_ISNANF_WORKS): Likewise.
* m4/isnanl.m4 (gl_FUNC_ISNANL_WORKS): Likewise.
* m4/ldexpl.m4 (gl_FUNC_LDEXPL_WORKS): Likewise.
* m4/log.m4 (gl_FUNC_LOG, gl_FUNC_LOG_WORKS): Likewise.
* m4/logf.m4 (gl_FUNC_LOGF, gl_FUNC_LOGF_WORKS): Likewise.
* m4/logl.m4 (gl_FUNC_LOGL_WORKS): Likewise.
* m4/log10.m4 (gl_FUNC_LOG10, gl_FUNC_LOG10_WORKS): Likewise.
* m4/log10f.m4 (gl_FUNC_LOG10F, gl_FUNC_LOG10F_WORKS): Likewise.
* m4/log10l.m4 (gl_FUNC_LOG10L_WORKS): Likewise.
* m4/log1p.m4 (gl_FUNC_LOG1P): Likewise.
* m4/log1pf.m4 (gl_FUNC_LOG1PF, gl_FUNC_LOG1PF_WORKS): Likewise.
* m4/log1pl.m4 (gl_FUNC_LOG1PL): Likewise.
* m4/log2.m4 (gl_FUNC_LOG2, gl_FUNC_LOG2_WORKS): Likewise.
* m4/log2f.m4 (gl_FUNC_LOG2F, gl_FUNC_LOG2F_WORKS): Likewise.
* m4/log2l.m4 (gl_FUNC_LOG2L_WORKS): Likewise.
* m4/logb.m4 (gl_FUNC_LOGB_WORKS): Likewise.
* m4/logbf.m4 (gl_FUNC_LOGBF_WORKS): Likewise.
* m4/logbl.m4 (gl_FUNC_LOGBL_WORKS): Likewise.
* m4/lstat.m4 (gl_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK): Likewise.
* m4/mbrtowc.m4 (gl_MBRTOWC_EMPTY_INPUT, gl_MBRTOWC_C_LOCALE): Likewise.
* m4/mkdir.m4 (gl_FUNC_MKDIR): Likewise.
* m4/mkstemp.m4 (gl_FUNC_MKSTEMP): Likewise.
* m4/mktime.m4 (gl_FUNC_MKTIME_WORKS): Likewise.
* m4/modf.m4 (gl_FUNC_MODF): Likewise.
* m4/modff.m4 (gl_FUNC_MODFF): Likewise.
* m4/modfl.m4 (gl_FUNC_MODFL): Likewise.
* m4/nanosleep.m4 (gl_FUNC_NANOSLEEP): Likewise.
* m4/perror.m4 (gl_FUNC_PERROR): Likewise.
* m4/popen.m4 (gl_FUNC_POPEN): Likewise.
* m4/posix_spawn.m4 (gl_FUNC_POSIX_SPAWN_FILE_ACTIONS_ADDCLOSE,
gl_FUNC_POSIX_SPAWN_FILE_ACTIONS_ADDDUP2,
gl_FUNC_POSIX_SPAWN_FILE_ACTIONS_ADDOPEN): Likewise.
* m4/ptsname_r.m4 (gl_PREREQ_PTSNAME_R): Likewise.
* m4/putenv.m4 (gl_FUNC_PUTENV): Likewise.
* m4/regex.m4 (gl_REGEX): Likewise.
* m4/remainder.m4 (gl_FUNC_REMAINDER): Likewise.
* m4/remainderf.m4 (gl_FUNC_REMAINDERF,
gl_FUNC_REMAINDERF_WORKS): Likewise.
* m4/remainderl.m4 (gl_FUNC_REMAINDERL,
gl_FUNC_REMAINDERL_WORKS): Likewise.
* m4/rename.m4 (gl_FUNC_RENAME): Likewise.
* m4/rmdir-errno.m4 (gl_FUNC_RMDIR_NOTEMPTY): Likewise.
* m4/rmdir.m4 (gl_FUNC_RMDIR): Likewise.
* m4/round.m4 (gl_FUNC_ROUND): Likewise.
* m4/roundf.m4 (gl_FUNC_ROUNDF): Likewise.
* m4/roundl.m4 (gl_FUNC_ROUNDL): Likewise.
* m4/signbit.m4 (gl_SIGNBIT): Likewise.
* m4/sleep.m4 (gl_FUNC_SLEEP): Likewise.
* m4/sqrtl.m4 (gl_FUNC_SQRTL_WORKS): Likewise.
* m4/stdint.m4 (gl_STDINT_H): Likewise.
* m4/strerror.m4 (gl_FUNC_STRERROR_0): Likewise.
* m4/strncat.m4 (gl_FUNC_STRNCAT): Likewise.
* m4/strtod.m4 (gl_FUNC_STRTOD): Likewise.
* m4/strtoimax.m4 (gl_FUNC_STRTOIMAX): Likewise.
* m4/strtok_r.m4 (gl_FUNC_STRTOK_R): Likewise.
* m4/trunc.m4 (gl_FUNC_TRUNC): Likewise.
* m4/truncf.m4 (gl_FUNC_TRUNCF): Likewise.
* m4/truncl.m4 (gl_FUNC_TRUNCL): Likewise.
* m4/tsearch.m4 (gl_FUNC_TSEARCH): Likewise.
* m4/tzset.m4 (gl_FUNC_TZSET_CLOBBER): Likewise.
* m4/ungetc.m4 (gl_FUNC_UNGETC_WORKS): Likewise.
* m4/unlink-busy.m4 (gl_FUNC_UNLINK_BUSY_TEXT): Likewise.
* m4/unlink.m4 (gl_FUNC_UNLINK): Likewise.
* m4/usleep.m4 (gl_FUNC_USLEEP): Likewise.
* m4/utimes.m4 (gl_FUNC_UTIMES): Likewise.
* m4/wcrtomb.m4 (gl_FUNC_WCRTOMB): Likewise.
* m4/wcsrtombs.m4 (gl_WCSRTOMBS_TERMINATION, gl_WCSRTOMBS_NULL):
Likewise.
* m4/wctob.m4 (gl_FUNC_WCTOB): Likewise.
* m4/chown.m4 (AC_FUNC_CHOWN): Add cross-compilation guess for native
Windows. Enable also on Autoconf 2.70.
* m4/printf.m4 (gl_PRINTF_SIZES_C99, gl_PRINTF_LONG_DOUBLE,
gl_PRINTF_INFINITE, gl_PRINTF_INFINITE_LONG_DOUBLE,
gl_PRINTF_DIRECTIVE_A, gl_PRINTF_DIRECTIVE_F, gl_PRINTF_DIRECTIVE_N,
gl_PRINTF_DIRECTIVE_LS, gl_PRINTF_FLAG_LEFTADJUST, gl_PRINTF_FLAG_ZERO,
gl_SNPRINTF_TRUNCATION_C99, gl_SNPRINTF_RETVAL_C99,
gl_SNPRINTF_DIRECTIVE_N, gl_SNPRINTF_SIZE1): Add cross-compilation guess
for native Windows.
(gl_PRINTF_POSITIONS, gl_PRINTF_FLAG_GROUPING,
gl_VSNPRINTF_ZEROSIZE_C99): Add comment.