meson: bump required version to 0.46

0.45 has a few annoying bugs (like the one in !358), and 0.46 is well over a year old by now (Apr 23, 2018), so let's move to it.

Note that all the distributions are well past this version, except for Ubuntu LTS which is still stuck at 0.45.1, but it makes no sense to let one slow distro prevent the rest of the world from moving on, especially when it's a trivial version bump on their side, and their users can install the current version using pip if they really need to build the latest mesa on the oldest ubuntu and can't update the meson package.

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@dbaker has also a bunch of commits waiting on this bump 🤗

/cc @asimiklit @idr