Quoting Gustavo A. R. Silva (2020-02-25 14:03:47)

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

I remember when gcc didn't support []. For the record, it appears
support for flexible arrays landed in gcc-3.0. So passes the minimum
compiler spec. That would be useful to mention for old farts with
forgetful memories.
-Chris