dpkg: Use utimensat() if available and in preference of lutimes/utimes

This funcion is only specified in POSIX.1-2008, but if it is, it is
going to be more standard than lutimes() which is only available on
Linux and the BSDs.

XXX: On recent Linux (2.6.32, 2009-12) and Solaris (11.1, 2013-04)
systems utimensat() is pretty much broken on multiple conditions.
→ But these issues (taken from gnulib and man utimensat(), do not really
affect our usage, no UTIME_OMIT nor UTIME_NOW, etc.

configure.ac

@@ -220,6 +220,7 @@ AC_CHECK_FUNCS([\
   getprogname \
   getexecname \
   lutimes \
+  utimensat \
   fallocate \
   posix_fallocate \
   posix_fadvise \

src/main/archives.c

@@ -482,6 +482,28 @@ static void
 tarobject_set_mtime(struct tar_entry *te, const char *path)
 {
   struct timeval tv[2];
+#ifdef HAVE_UTIMENSAT
+  struct timespec ts[2];
+  int rc, flags;
+
+  ts[0].tv_sec = currenttime;
+  ts[0].tv_nsec = 0;
+  ts[1].tv_sec = te->mtime;
+  ts[1].tv_nsec = 0;
+
+  if (te->type == TAR_FILETYPE_SYMLINK)
+    flags = AT_SYMLINK_NOFOLLOW;
+  else
+    flags = 0;
+
+  /* Try to use the POSIX.1-2008 interface, and fallback to the old code in
+   * case it's not supported by the system at run-time. */
+  rc = utimensat(AT_FDCWD, path, ts, flags);
+  if (rc == 0)
+    return;
+  else if (rc < 0 && errno != ENOSYS)
+    ohshite(_("error setting timestamps of '%.255s'"), path);
+#endif
 
   tv[0].tv_sec = currenttime;
   tv[0].tv_usec = 0;