Merge branch 'build-infrastructure-improvements' into 'master'

Build infrastructure improvements

This merge improves the robustness and the utility of the Athena build scripts.

For the `build_externals.sh` script if an install of AthenaExternals and Gaudi is found then the script will exit, unless `-f` is specified. This allows the script to be called multiple times safely, e.g., by the continuous integration system. Specifically the check is done against installs of AthenaExternals and Gaudi, so failed builds should not cause the script to think that there is a valid installation. There were minor improvements to use absolute paths internally that make debugging easier.

In the main `build.sh` script, I factorised out the environment setup piece to `build_env.sh`, allowing it to be easily sourced when someone wants to manage the build themselves or invoke the build commands in special ways.

There is also now support for options in the main build script to control which pieces of the build are done (cmake, make, install, cpack) allowing more detailed control (e.g., the continuous integration system certainly doesn't care about making RPMs - @cgumpert will like that).

Finally, there is a small change to the main `CMakeLists.txt` file to remove the zero compression option from RPM generation (which only saves a small amount of time, but inflates the RPM size greatly).

See merge request !177