On NextStep, <utime.h> doesn't define struct utime, unless we

On NextStep, <utime.h> doesn't define struct utime, unless we
#define _POSIX_SOURCE before #including it.
* aclocal.m4 (GUILE_STRUCT_UTIMBUF): New test.
* acconfig.h: New comment text for above CPP symbol.
* configure.in: Call it.
* posix.c: #define _POSIX_SOURCE if it seems necessary.

* configure.in (AC_CHECK_HEADERS): Include sys/utime.h and utime.h
in the list.
* posix.c: Check HAVE_SYS_UTIME_H and HAVE_UTIME_H, instead of
testing for __EMX__.

* posix.c: #include <libc.h>, if it exists.

* posix.c: Cast the return result to GETGROUPS_T, not gid_t; we
don't even know if the latter exists.

* posix.c (s_sys_setpgid, s_sys_setsid, s_sys_ctermid,
s_sys_tcgetpgrp, s_sys_tcsetpgrp): Renamed from s_setpgid,
s_setsid, s_ctermid, s_tcgetpgrp, s_tcsetpgrp, for consistency.

* posix.c (R_OK, W_OK, X_OK, F_OK): #define these if the system's
header files don't.
(scm_init_posix): Use them when initializing the Scheme constants
of the same name.