Adds an option to build libsodium from source instead of using the
system-provided one.
This is useful on systems that do not provide libsodium, or for testing
changes to libsodium along with devilutionx (by specifying
`-DFETCHCONTENT_SOURCE_DIR_LIBSODIUM`).
This also allows us to configure sodium to our liking when building it from source,
such as `SODIUM_MINIMAL`, which reduces the x64 release binary size from 4.3 MiB to 4.1 MiB.