BUILD: Add a DragonFlyBSD target

Add a target for DragonFlyBSD 4.3 and above.
diff --git a/INSTALL b/INSTALL
index f562a71..0263cf3 100644
--- a/INSTALL
+++ b/INSTALL
@@ -376,6 +376,7 @@
   - linux-musl          for Linux kernel 2.6.28 and above with musl libc
   - solaris             for Solaris 10 and above
   - freebsd             for FreeBSD 10 and above
+  - dragonfly           for DragonFlyBSD 4.3 and above
   - netbsd              for NetBSD
   - osx                 for Mac OS/X
   - openbsd             for OpenBSD 6.3 and above