cli: preserve sub-second mtime on the BSDs (fixes #1515)

HAVE_UTIMENSAT was only set when _POSIX_C_SOURCE >= 200809L or
_ATFILE_SOURCE was defined. FreeBSD, OpenBSD, NetBSD and DragonFly BSD
provide utimensat()/futimens() and nanosecond-resolution stat timestamps
by default without defining either macro, so the CLI fell back to utime()
and truncated the copied mtime to whole seconds (visible as .br files with
a .000000000 nanosecond field next to the source).

Detect those platforms explicitly. The existing non-Apple ATIME_NSEC /
MTIME_NSEC macros already use the st_atim / st_mtim spelling that the BSDs
expose, so no further change is needed.

c/tools/brotli.c

diff --git a/c/tools/brotli.c b/c/tools/brotli.c
index 98ff0a4..5554ced 100644
--- a/c/tools/brotli.c
+++ b/c/tools/brotli.c
@@ -79,6 +79,9 @@
 #define HAVE_UTIMENSAT 1
 #elif defined(_ATFILE_SOURCE)
 #define HAVE_UTIMENSAT 1
+#elif defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__) || \
+    defined(__DragonFly__)
+#define HAVE_UTIMENSAT 1
 #else
 #define HAVE_UTIMENSAT 0
 #endif