Utils: Add macro for C++ default arguments in C headers

This macro allows defining a default argument for when the translation
unit is compiled in C++. Otherwise (in C), the argument has to be passed
explicitly.

A couple of benefits:
* Default arguments are a nice quality-of-life feature in C++. It's
  annoying if these can't be used in C++ files, just because the header
  with the function declaration still needs to be C compatible.
* Adds useful information to the API declaration. E.g. that an argument
  can be nullptr.
* Should help us to move to using default arguments more, helping
  readability (arguably)

Used in D14653.

Reviewed By: LazyDodo

Differential Revision: https://developer.blender.org/D14654

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