bsys: Meson build system

[obiwac]

One part of #28

• The only thing I still wanna do is write out the BSYS-specific path to some global variable, cuz I don't want to run the risk of someone not knowing about where this should be and putting it somewhere inconsistent instead.
• Oh and also better tests for the dep-tree return values (maybe just pull these out into their own PR).
• Almost forgot, maybe check that the Meson/Ninja commands exist before we do anything? Cuz sometimes setup takes a long time and it'd suck to wait for setup to finish only to be told that we're missing Ninja.