meson: Fix DragonFly BSD identifier in meson build files

This changes the operating system identifier tested against host_machine.system() in meson build files from "dragonflybsd" to the officially stable "dragonfly" as detailed here: https://mesonbuild.com/Reference-tables.html#operating-system-names

Thank you for reading this. If there is anything I could improve or clarify, please let me know.

Signed-off-by: George Matsumura gmmatsumura01@bvsd.org