Do not force USE_INTRINISCS to be set via source-code

[coldtobi]

One important part of #228 was unfortunatly reverted one commit later :(

The problem is, that without those line USE_INTRINSICS cannot be "unset" via the CMake option; If unset, it will be set by the code, which leads bottom line the option always set and not configurable by CMake on

Please note that USE_INTRINSICS defaults to "defined" (see CMakeLists.txt), so the behaviour is the same unless the user overrides this (and with the lines in the code he cannot override it)

Tobi

[RobertBeckebans]

Ah your patch disabled intrinsics with MSVC on Windows. I'll make another commit that fixes CMake