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

* refs/heads/t/build:
  build: Add $(LDFLAGS) to rule for oslfsck.
  build: Prefix CFLAGS and friends with OSL_.
  build: Reorder make variables.
  build: Combine CFLAGS and LIBCFLAGS.
  build: Simplify LDFLAGS setting.
  build: Simplify Makefile.
  build: Drop support for SunOS and MacOS.
  build: Don't insist on gcc.
  build: Create dependencies on the fly.