Use utimensat() on FreeBSD

FreeBSD only claims to support POSIX 2001 [0]. But they do in fact support
`utimensat()`. This adds a specific check to opt them in to using it. This
value was selected by consulting [1].

See discussion on facebook#3952.

Further addresses facebook#3748.

[0] https://github.com/freebsd/freebsd-src/blob/937a0055858a098027f464abf0b2b1ec5d36748f/sys/sys/unistd.h#L96

[1] https://docs.freebsd.org/en/books/porters-handbook/versions/

programs/util.c

@@ -259,7 +259,8 @@ int UTIL_utime(const char* filename, const stat_t *statbuf)
      * that struct stat has a struct timespec st_mtim member. We need this
      * check because there are some platforms that claim to be POSIX 2008
      * compliant but which do not have st_mtim... */
-#if (PLATFORM_POSIX_VERSION >= 200809L) && defined(st_mtime)
+#if (PLATFORM_POSIX_VERSION >= 200809L) && defined(st_mtime)) \
+    || (defined(__FreeBSD__) && __FreeBSD_version >= 1100056)
     {
         /* (atime, mtime) */
         struct timespec timebuf[2] = { {0, UTIME_NOW} };