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
readme: fix typo
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
Fix include of OpenGL headers on OS X systems

On OS X systems, the OpenGL headers are located in an "OpenGl" folder
instead of the "GL" folder they're located in in most other systems.
buildsystem: added --raw-cmake-args to ./configure helper

allows specifying arguments that are passed directly to cmake
buildsystem: fixed pep8 issue in ./configure
Merge pull request #13 from rossmurray/patch-1

readme: fix typo
Merge pull request #15 from svkurowski/fix_gl_includes_os_x

engine: fix include of OpenGL headers on OS X systems
util: introduced compiletime rotate left function
ci: notify messages enabled for irc bot
audio: fixed missing includes in audio/types.h
Update milestones.md - Minor typo

applicatin -> application
Merge pull request #17 from aw3stin1/patch-1

doc: update milestones.md - Minor typo
readme: added flattr button