Set the source version number in a clean fashion

Set the source version number in a clean fashion

This commit changes the version number of the tarball created by 'make
dist'. One can set PDNS_BUILD_NUMBER to create incremental version
numbers. This commit also adds a message to secpoll about PowerDNS being
from git and not a released version.

The version produced depends on the branch:

master: 0.0.${PDNS_BUILD_NUMBER}g${GIT_HASH}
rel/*: ${PREVIOUS_TAG}-${NUM_COMMITS_AFTER_TAG}-${GIT_HASH}
any other: 0.0.${BRANCH_NAME}.${PDNS_BUILD_NUM}g${GIT_HASH}