Fix for issue #178

When doing G_RunThink for NPCs, it will skip the NPC_Think function and only run icarus when it hasn't reached the nextthink time.
Which means that the NPCGlobals such as NPCInfo will not be set for this run of icarus.
This causes null pointer crashes when icarus needs to access the npc globals.
Merge pull request #183 from xScooper/master

Fix for issue #178
Fix: hardcoded pointer size, segfault on 64 bit Linux
Fix: added missing _isnan macro for Linux
Change: minor modification for qsort parameter
Added a PROPER section for buying the game

Added a few links to places where you can obtain the game, finally figured out how to do it.
Merge pull request #185 from liamdawe/patch-1

Added a PROPER section for buying the game
Merge pull request #184 from exander77/master

Fixes for building on 32 and 64 bit Linux
Added a few helper macros, which I will be using later on.
Adds the "forcesight" surface type to MP.
Currently the renderer skips them for now until RDF_ForceSightOn renderflag is implemented.