Add support for Meson build system.
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
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