small updates (hopefully)

- bin/audio/*: added demos datas, so user does not have to download them from website
- demos/*: missing stdio headers
- demos/welcome: another non-SDL sample added
- tools/resamplerlab/main.cpp: uninitialized variables, also bugfixed typo (compiler warnings)
- build/premake4.lua: moved locals to top in case user wants to tweak them (easier to figure out)
- build/premake4.lua: default settings produce non-SDL/native-only backend config (no dependencies build)
- build/premake4.lua: do not compile SDL samples if no SDL is found (build errors)
- build/premake4.lua: 3dtest demo name typo (never compiled)
- build/premake4.lua: try to detect DXSDK_DIR as well (less hardcoded)
- AUTHORS: update