CMake: several improvements

[SpaceIm]

• add IIR1_BUILD_DEMO option to disable build of demo executables
• rename INSTALL_STATIC option to IIR1_INSTALL_STATIC
• allow IIR1_BUILD_TESTING to disable tests even when iir1 is top project
• move cmake helper files under cmake folder
• use cxx_std_11 compile features instead of CMAKE_CXX_STANDARD (so that consumers can use another more recent standard), and therefore bump CMake min version to 3.8
• no warnings as errors
• remove useless stuff in demo & test (no need to declare again project(), cmake_minimum_required(), flags already injected globally, include dirs of iir since target is already linked to executables)

[berndporr]

Sorry there are just too many changes to cmake in one PR and it changes a lot. Not good to review and hard to de-tangle.