Use `make STATIC=1` to build a static library.
Some language ecosystems, like those of Rust, have a convention of
static linking in order to make binaries easier to distribute. In those
environments, this feature helps libui fit in more easily.
In order to prevent internal symbols from linking, we first create an
intermediate object file with `ld -r` and strip private symbols from it
before using `ar` to create the library.
Patrick Walton