Consider making Boost-Test optional

[nilsdeppe]

It seems like I need Boost-Test installed on my machine in order to build the library at all. This isn't a huge deal, but Boost-Test seems to be one of those libraries that has a different name in lib form libboost-test, while in the Boost documentation it is called unit test. I think there are two things that would make life a bit easier:

• state in the readme "Boost libraries ('libboost-test` for testing)"
• if Boost-Test cannot be found, disable building the tests and maybe print a warning message during meson runtime

[wokast]

Thanks for pointing this out. Boost is indeed treated as a monolithic all-or-nothing dependency at the moment, mainly because the partitioning into subpackes seems to vary. However, failing to satisfy the test build dependencies should not prevent the main library from building. I intend to fix this soon.