Merge branch 'refs/heads/t/makefile_conventions'

A bunch of changes which strive to make the paraslash build system
more standard.

Cooking for three months.

* refs/heads/t/makefile_conventions:
  build: Add target "check" as a synonym for "test".
  build: Rename target "tarball" to "dist".
  build: Do not strip installed executables by default.
  build: Support $(DESTDIR).
  build: Rename clean targets.