build: reduce complexity for setting the version (#1466)

This change refactors the implementation of how the version is embedded
in the binary to reduce the number of variables necessary to determine
the version information from 3 to 1.

The legacy build variables are still supported, however, a warning will
be emitted instructing users to contact their package maintainer. The
legacy GitExacTag variable, if present, will be used to set main.version
if it is undefined. This ensures that unmigrated package builds will
continue to provide the correct version information.

The legacy build variables will be supported until 0.12.0, giving
package maintainers some time to migrate.

Change-Id: I05fea97169ea1af87b198174afe5b6663f860fd8