liblightning needs to be able to be linked statically

[kulp]

Since #10 switched to dylib linking for liblightning, there is a problem -- an external user of lightning-sys will not know where to find the dynamic liblightning (such as liblightning.so.1 on Linux, or liblightning.dylib on macOS), because it exists only in the crate build directory. This can manifest like this:

<name-of-binary>: error while loading shared libraries: liblightning.so.1: cannot open shared object file: No such file or directory

For example, even cargo tarpaulin does not find liblightning.so.1 unless it happens to be installed on the host computer already.

This was an unintended consequence of #10. I think static linking would be best, for multiple reasons, but I do not yet have an idea how to make it work given the issues raised in e9301f4.

We could reinstate the workaround from c465a46 (removed in 522ee66), but that is a bit of a hack.

Originally posted by @kulp in #21 (comment)