Initial libsodium build automation

RbNaCl is (at least temporarily) goint to be targeting libsodium as the
goal is to produce a Ruby FFI binding, which requires a shared library,
and the official distribution of NaCl does not presently build shared
libraries.

In addition, libsodium is all portable C code and can build on Windows
under MinGW. By using it, RbNaCl should be able to support all
combinations of Ruby interpreters (e.g. MRI, JRuby, rbx) on all
operating systems, including Windows.