Add pkgconfig output

[RossSmyth]

Thanks for this cool library.

This slightly reworks the meson build, primarily so that it can generate a pkgconfig file for ease of integration.

1. Disable building tests by default. Now they will only build when meson test is called
2. Only build the pkgconfig and tests when it is not a subproject. Consumers won't care if it's a subproject.
3. Configure the amalgamation file with a *.c because Meson just ignores it otherwise
4. Build the library (up to user what type)
5. Install the header
6. Generate the pkgconfig file

I do not know what Meson version you are targeting, so I just said 1.0. I also ran meson fmt on the top-level file.

[sammycage]

Interesting. Thank you for the wonderful contribution. Since the Meson build is mainly intended for tests and examples, I prefer to keep it minimal for now. Out of curiosity, what is your typical use case for needing a pkg-config file with this kind of library?

[RossSmyth]

As stated, it makes it easier to compose the library in a program with other libraries, rather than needing to worry about specifics of whatever the library needs. Other course it means adding the header to the compiler search path, and adding the compiler flags needed.

[RossSmyth]

It would be nice to package this library once it is ready to nixpkgs, so this is a good step for that as pkgconfig is mostly required in nixpkgs.