New file. Extracted from sh-utils' configure.ac

and extended to work also with *BSD systems.

m4/boottime.m4

@@ -0,0 +1,37 @@
+# Determine whether this system has infrastructure for obtaining the boot time.
+
+# GNULIB_BOOT_TIME([ACTION-IF-FOUND[, ACTION-IF-NOT-FOUND]])
+* ----------------------------------------------------------
+AC_DEFUN([GNULIB_BOOT_TIME],
+[
+  AC_CHECK_FUNCS(sysctl)
+  AC_CHECK_HEADERS(sys/sysctl.h)
+  AC_CACHE_CHECK(
+    [whether we can get the system boot time],
+    [gnulib_cv_have_boot_time],
+    [
+      AC_COMPILE_IFELSE(
+       [AC_LANG_PROGRAM(
+[AC_INCLUDES_DEFAULT
+#if HAVE_SYSCTL && HAVE_SYS_SYSCTL_H
+# include <sys/param.h> /* needed for OpenBSD 3.0 */
+# include <sys/sysctl.h>
+#endif
+#ifdef HAVE_UTMPX_H
+# include <utmpx.h>
+#else
+# include <utmp.h>
+#endif
+],
+[[
+#if defined BOOT_TIME || (defined CTL_KERN && defined KERN_BOOTTIME)
+/* your system *does* have the infrastructure to determine boot time */
+#else
+please_tell_us_how_to_determine_boot_time_on_your_system
+#endif
+]])],
+       gnulib_cv_have_boot_time=yes,
+       gnulib_cv_have_boot_time=no)
+    ])
+  AS_IF([test $gnulib_cv_have_boot_time = yes], [$1], [$2])
+])