We now depend on libsodium unconditionally. Future work will require
functions from libsodium, and nobody we're aware of uses the nacl build
for anything other than making sure it still works on CI.
We want to use the same libtool style .so versions in both build systems,
ideally both systems should read the version information from the same
configuration file.
This commit introduces an so.version configuration file and sets up
the autotools to use it.
The version numbers in so.version define the ABI compatibility and should be
updated prior to each release.
implements #323
