Use a different build directory for each target / test

Because all builds are performed in the `./build` directory, when building different targets without cleaning, the second time the project is build, the older build files are found and might be invalid for the new target. Building each target in its own directory should improve user experience, avoiding unnecessary cleans (for example, when going back and forth between tests and cross compilation).

Start by tracing where the current ./build directory is selected for target and test builds. Check how target identity and cross-compilation settings are represented, then verify that each target uses its own directory and that switching between tests and cross compilation works without cleaning.

Written by the indexing model from the issue text.