Don't include the lazy precompiled.h everywhere, only what's
required for the compilation unit.
platform.h needs to be included instead to provide all essential
defines and types.
All includes use the relative path to the neo or the game
specific root.
Move all idlib related includes from idlib/Lib.h to precompiled.h.
precompiled.h still exists for the MFC stuff in tools/.
Add some missing header guards.
Protect all SIMD implementations with the according defines and
let the compiler decide if it supports the intructions.
Linux will still use Simd_Generic because CPU feature runtime
detection is missing.
