- Install ninja in pipelines
- Only show warnings in the GitLab UI; put all other output in a file so
  it doesn't make the logs hard to read.
- Refactor out a `compile_octo` function.
- Change some mentions of 'make' to 'ninja'.
- Document ninja in the readme
- Install ninja from github releases on CentOS

  It's not packaged on CentOS 7.

- Clean up a few comments

- Allow relative paths in .ref files

  + Allow any `./` to start a path instead of requiring absolute paths
  + Make PATH regex more selective

    Previously, it replaced `[127.0.0.1] ../src/octo.c` with `[127PATH`,
    which was clearly incorrect. The new regex requires a space before the
    filename, which works in practice while still allowing spaces within
    filenames.

    This also uses the regex consistently, even when `verify_helper` is not
    used directly.

- Allow using `ninja` with docs.sh

  This allows using either Make or Ninja by detecting the `Makefile` in
  the build directory. If both a `Makefile` and `build.ninja` are
  precedence, make will be given precedence.

- Allow using either Make or Ninja in the pipeline

  This allows testing that `make` still works without having to revert all
  the Ninja changes.