The build_cmake.sh script is a wrapper that does 3 things:

• it cleans the build directory
• it runs cmake to configure the project
• it runs ninja on the selected target, all by default

Since the CI build are migrating to descriptive builds, the last 2
pieces are now part of the configuration. The added value of the script
is only the call to git clean to clean the build directory.
This diffs make the builds defined fromt the configuration be
independent of the `build_cmake.sh' script. Once all the builds are
migrated, the wrapper can be completely removed.

Depends on D7380.