Makefile: add ".NOTPARALLEL:"
Several of the dependency lists say "clean-build ${BUILD_DIR}/_build",
which is intended to clean the build dir, then recreate it. Of course, if
the user has make
set to build dependencies in parallel, then that won't
work.
Now, I'm not a fan of putting "clean" as a dependency of a constructive step. It's sloppy. But hey, it's not my project.