use incremental linking in debug mode

reduces linktime in windows from >15s to about 1s by removing "whole program optimization" and "link time code generation" (/GL /LTCG) from debug mode. currently the "whole program optimization" doesn't work in release mode too as I couldn't find a way to declare it for release and debug differently.

I changed the way glow is included. previously the main CMakeList.txt was included, now I only include glow/source/glow and glow/source/glowutils to prevent the cmake dir to be added which overrides some defines from the voxellancer/cmake dir