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

source/blender/blenlib/BLI_utildefines.h

@@ -812,6 +812,16 @@ extern bool BLI_memory_is_zero(const void *arr, size_t arr_size);
 #  define ENUM_OPERATORS(_type, _max)
 #endif
 
+/**
+ * Utility so function declarations in C headers can use C++ default arguments. The default is then
+ * available when included in a C++ file, otherwise the argument has to be set explicitly.
+ */
+#ifdef __cplusplus
+#  define CPP_ARG_DEFAULT(default_value) = default_value
+#else
+#  define CPP_ARG_DEFAULT(default_value)
+#endif
+
 /** \} */
 
 /* -------------------------------------------------------------------- */