Multiple builds from same source tree

[hiker]

During my meetings in NIWA a point was made that often you might want to compile the same source version with different options or compilers (e.g. CI or so). ATM, this requires two complete checkouts of the sources (in .../source), which then also get copied into build_output, so we have 4 copies.

We should investigate if we can have different builds based on the same sources in one project directory, e.g. by giving a suffix to build_output (e.g. build_output_ifort, build_output_optimised, ...).

We would then either give the same suffix to the name of the binary, or we store the binary in the build_output directory.