Generate ngIRCd version number from GIT tag

Generate ngIRCd version number from GIT tag

Now the ngIRCd release/version number is deduced from the "current"
annotated GIT tag; see "git describe --help" for details. This is the
same scheme the Linux kernel uses and gives much more details version
numbers for interim releases and inofficial source archives generated
using "make dist".

Please note: the version number is only updated it the autogen.sh
script is run; so after pulling in and pushing out new commits, you
should run ./autogen.sh!