Support Meson about opus

[rillian]

So in general we're happy to take contributions for other build systems if they're helpful to downstream library users. The main issue is ongoing maintenance.

• The core development team doesn't have much time to maintain all the different build systems, so if there isn't a community of people available to keep it running, we'll just have to remove it again, which is disappointing for users.

• We have a single source for the list of source files, in *_sources.mk and *_headers.mk that the current build systems use, except for Visual Studio. We'd like any new additions to build on those, to minimize the number of places people adding/removing source files have to update.

If you can address both of those, then a meson config, with integration tests so we know when it breaks, is fine. I don't know much about meson, I guess you'd have to shell out to a custom generator script or upstream a makefile module to address the second?

Note there's already a PR open to add CMake support, and VS2017 has native CMake build support, so that will probably be more immediately popular if you just want an alternative to autotools.

[Timmmm]

Good points. Unfortunately it doesn't seem like it is possible to read & parse a makefile from Meson like that at the moment.

Given that Meson is probably not at the point where I would have it as the only option for a project like this, and you don't want to support more than one build system, I would say entirely moving from Autotools to CMake is probably the best option. True, CMake is crazy, but not quite as crazy as Autotools, and it works quite well on Windows.

[andreaskem]

I took a stab at this out of boredom:
https://github.com/andreaskem/opus/commit/f7ade90143e58078ed5e342a136516e6996156a3

There are several caveats mentioned in the commit message so this might not be useful for anybody. Provided without warranty, your mileage may vary, yadda yadda yadda.

[Timmmm]

Nice, though I think using Python to parse the .mk files shouldn't be part of the meson.build. Probably would be better as a pre-generation step that writes a meson.build file with the correct filenames.

Also you probably want to use target_machine in most places you have used host_machine.