Support bulding on musl libc

There are a couple of things that don't work when building under musl,

• The linux/if_alg.h header isn't available (even though __linux is defined)
• The be64toh() / htobe64() macros are defined, but the existing test didn't work because it looks for glibc version

My recipe for getting a static build using musl rather than glibc is to configure librist with,

CC=musl-gcc meson -Dbuiltin_lz4=true -Dbuiltin_cjson=true --default-library=static