Merge branch 'ab/coding-guidelines-c99' into maint-2.38

Update CodingGuidelines to clarify what features to use and avoid in C99. * ab/coding-guidelines-c99: CodingGuidelines: recommend against unportable C99 struct syntax CodingGuidelines: mention C99 features we can't use CodingGuidelines: allow declaring variables in for loops CodingGuidelines: mention dynamic C99 initializer elements CodingGuidelines: update for C99