Embed source and build info into the version string

Most places now show the source VERSION, plus three new details:
* DIST_HOST (where the sources were built)
* BUILD_DATE (when the binaries were built)
* BUILD_HOST (where the binaries were built)

Some places only use VERSION, but it's not clear if they would benefit from the additional data.
Also, embedding this data into version.bind/version.pdns might not be a good idea, but it's in the code right now.