Report MIC SIMD correctly

Avoid cmake warning because SIMD cannot be detected. And
avoid that version information prints "None". This is
needed because MIC doesn't use GMX_SIMD so that the same
setting (__MIC__) can be used both for native and (future)
offload.

Also added installguide section.

Change-Id: I4af3b13678aa5c3c69c60bda499c3691ac83dc53