We can, so let's not ask if we should!
More realistically, we can right now so it's nice to mark that we do, but this is subject to be re-evaluated if any later changes conflicts with this.
Note also that this only affects the binaries we distribute with our releases!
It may very well be possible to compile RGBDS on even older versions of Mac OS X if you have the appropriate compiler setup;
we're just not testing it nor offering such binaries ourselves, but third-party packagers are welcome to do so.
(Looking at you, TigerBrew! ;)
Automatically grabs and compiles libpng, and avoids having hacky lines in our Makefile
(the compiler specification *should* be orthogonal to the build target!)
Also move the MinGW package install to the `install_deps.sh` script,
to move logic off of the YAML.
Besides the immediate performance improvement, reducing
the frequency of downloads should make spurious
failures (network, rate-limiting...) less bothersome.