Meson build support

I made this for the Meson WrapDB but I wanted to offer it to the upstream project first. It should be faster than the autotools build, and allows for multiple configurations to be built in separate directories when developing. It also allows the library to be built as a subproject for applications that make use of it.

Build is tested across the major platforms (Linux, macOS, and Windows, both native MSYS2 and cross-compile) as well as the BSDs, OpenIndiana, and Haiku. Where possible I've tried to preserve the behavior of the autotools build with the default settings, except for a few improvements such as supporting Java 18+. I've also ensured that the symbols exported by the shared library are the same, at least on Unix. Please let me know if you have any questions or there's anything I need to change.