New/updated config file options:

* 'configure-flags', reused as the way to pass cmdline options to the
meson setup command.
* 'ninja-options', a new option to pass cmdline options to the `ninja`
command. Note that ninja is mandated by Meson as the underlying build
tool.

Tested with https://github.com/plibither8/2048.cpp

Fixes #27, reviewed in !8.

Test suite passes and I continue to be able to build 2048.cpp. I've also
validated that ninja-options is passed to ninja when building 2048.cpp,
though this was a manual verification.

CCBUG:406268