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