Preprocessor symbls are generally unable to be set (with Meson)

[rsmyth-ecmi]

When building Unity with Meson, one is unable to set symbols for configuration. The main one is UNITY_FIXTURE_NO_EXTRAS. But there is a whole laundry list in https://github.com/ThrowTheSwitch/Unity/blob/cdf1d0297effc2736ee847e557b4275b4f02310b/docs/UnityConfigurationGuide.md that could be included as well, as I would expect most are global to the target platform.

The alternative is setting CPP symbols before importing the header but that's not the best way of config imo.

[rsmyth-ecmi]

#769 Adds a couple. More can be added.