Make it possible to build Gaudi for ATLAS with Ranges v3 support

We would like to start to use the functional code developed for Gaudi recently. Unfortunately using the current Gaudi code we were unable to do so, as the ATLAS build environment doesn't add the LCG externals to CMAKE_PREFIX_PATH in the environment. Instead we just extend the CMAKE_PREFIX_PATH CMake variable when/if necessary.

With this small re-ordering of the main project file it becomes possible to set up the environment correctly for the cmake/externals.cmake code to find the Ranges v3 headers when building Gaudi on top of the ATLAS externals project.