build: ensure we aren't using GNU extensions

Summary:
Remove the use of gnu extensions:

• anonymous structs
• variadic macros with no argument

And add a Clang warning for detecting their future use.

Backport of core PR18088.

Test Plan:
With clang:

ninja all check

Check there is no warning.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9109