SCons: Implement putting git version info in the docinfo.xml

I had to turn off the "don't duplicate" setting for VariantDir to have the docinfo.xml
included by the main ".asc" file (because AsciiDoc requires it to be in the same dir)
and therefore had to change some dependencies from being under "src/"
to under "build/".

Also, removed last hard-coded reference to "Discordians", using "${PROJECT_NAME}" instead.