Windows is built with a seperate descriptor to handle additional changes
that need to be done to the end binary. Consolidate the gitian-build
script for this change.
The signature prepare tool and the gitian-builder git repo should be
checked for their content. For this purpose, checkout the gitian-builder
repo at a specific commit and take the sha256sum of the osslsigncode
tool.
This adds a build script to run gitian builds for linux.
The build script was copied from bitcoin and then adapted for monero.
Build step documentation is outlined in the README in the contrib/gitian
directory.
