buildsystem: improved VERBOSE in Makefile

if VERBOSE, make will now print 'entering directory' messages.
buildsystem: fixed minor issue with doxygen

dot executable wouldn't be inserted correctly due to a scoping issue
doc: added buildsystem documentation

this doc has been optimistically linked by `building.md` for some time, but was non-existant thus far.

also fixed the link in `building.md`.
Merge branch 'master' of git.sft.mx:openage
buildsystem: removed stray `TODO`

whee! only 88 of them left now!
Update building.md
Merge branch 'master' of https://github.com/snoek09/openage
buildsystem: fixed cmake 2.8 support for python 3.4

thanks to organman91 for reporting

fixes #12