Update minimum required version of CMake to 3.9

The main reason for this update is to use COMMAND_EXPAND_LISTS in
add_custom_target() and add_custom_command(), which lets us pick
include directories needed for rootcling from the library target
that needs a dictionary to be generated. This has been introduced
in CMake 3.8. The other reasons are that starting with CMake 3.8
the try_compile() command respects CMAKE_CXX_STANDARD if it is set
and support for CUDA as a first class language. CMake 3.9 adds IPO
support and the DISABLED property for tests, which we want to use.
It also has better modules for GoogleTest and Doxygen support, as
well as full support for CUDA on Windows and the ability to install
or export object libraries. Please see the release notes for more
information: https://cmake.org/cmake/help/latest/release/index.html