Don't instanciate variable if it doesn't exist

see line 88 where `xcontext` is declared only if `GemGlewXContext` is defined
forgot that one...
link [gemcocoawindow] against cocoa-framework

Closes https://github.com/umlaeute/Gem/issues/99
(well, hopefully; i couldn't test)
added travis-ci scripts

modified versions from avilleret (thanks!)
- scripts now live in build/travis-ci/
- builds are out-of-sourcetree (in build/travis-ci/${OS}-${ARCH})

TODO:
- move dependency-installation into separate scripts
added some debugging
install OSX-dependencies
install linux dependencies
dependency install-script
removed dep-installation from build-script
call depinstall.sh
cd to root-directory before running autoreconf
fixed typo
fixing more typos
moved sudo-parts into separate script
check for return-value of makeCurrent()

Closes: CID#75581
don't allocate unused memory

Closes CID#75582, CID#75585
initialize VertexBuffer::type

Closes CID#75583, CID#75587, CID#75589
Initialize model::m_size_change_flag

Closes CID#27971
initialize GEMglFogfv:pname

use GL_FOG_START, as this seems to be a no-op
mark unterminated "default" as intended

Closes CID#75591