Issue errors/warnings for ICC before 12.0.0

ICC 11.1 with SSE4.1 gives incorrect results, so CMake will
refuse to build with that minor version. Given that we
currently don't know anything specific and 11 is old, we
warn with any version older than 12.0.0. If more information
is available later, this warning can be made more specific.

Fixes #1126

Change-Id: I3c37844c69ce34769187f288c43cd8aef4086c53