Build OpenSSL instead of using distribution-provided library.
Instead of using the boost provided by Ubuntu 12.04, build our own dependency like we do for Windows. This allows using a much newer version (1.55 versus 1.46) as well as building with `-fPIC` so that `-pie` can be used in the x86-64 build.
Change Linux deps to use a zip archive rather than a gzipped tarball to match win32 Rename Linux descriptor to gitian-linux.yml to match win32