Improve Meson build with configuration

[concatime]

Plus some minor changes.

Would you like to add a .clang-format in the repo? Something simple, like

AllowShortIfStatementsOnASingleLine: true
BasedOnStyle: Microsoft
ColumnLimit: 80

[randy408]

The SPNG_STATIC define is the only macro that is sometimes required before including spng.h, even that is only needed on Windows, the rest are compile-time options and have no effect inside a config.h. When the Meson or CMake project is used it's defined automatically, I don't think it makes sense to have it in a header, especially if the library has both static and dynamic builds.

With that in mind, I'm against a config.h and moving spng.h into a separate directory.

I haven't considered a .clang-format, there aren't that many contributors.

[concatime]

Understood. Give me some time and I will rework my patch.