Optimize the way of generating XDR classes.

[overcat]

What problem does your feature solve?

dependent on manual execution of xdrgen, this offline step may be error-prone and does not lead to reproducible builds, since the commit hash/versions used at generation time for xdrgen and the xdr files themselves are are not captured.

See #498 (comment)

What would you like to see?

Add a Makefile build script similar to this on: https://github.com/stellar/go/blob/master/Makefile

What alternatives are there?

N/A