Merge branch 'refs/heads/t/build' into next

* refs/heads/t/build:
  build: Deactivate implicit build make(1) rules.
  build: Mark files generated by lopsub as precious.
  build: Remove make(1) targets to build a static library.
  build: Combine make(1) targets osl_errors.h and osl.h.
  build: Make sure osl.h exists when we run the compiler.