make build reproducible

Currently looking at this. Possible change for version 7.4.

this functionality has been implemented in the upcoming 7.4 release. The feature was implemennted as follows.

• the SOURCE_DATE_EPOCH variable can be set to fix the value of timestamps included in build artifacts. this allows users to build DC3DD in a reproduceable way.
• If the user wishes to go a step further and replicate DC3DD's build used for testing the final release, A file named reproduceable.yml is included in the root of the source directory containing information about the build environment used by DC3 for building and testing the final release. This data should allow you to reproduce DC3's build if desired.
• Two files, digests.sha512 and digests.b2 are located in the root of the source directory. these contain SHA-512 and Blake2B hashes of the dc3dd binary and the dc3dd.1 man page, as produced by DC3's build process. The hashes in these files are identical to those in reproduceable.yml; the digest files are included to facilitate easy verification using sha512sum and b2sum utilities.
• A file named README .reproduceable in the root of the source directory describes in detail how to perform reproduceable builds of DC3DD.