Avoid flexible arrays on unsupported platforms via a macro.

[kareem-wolfssl]

Description

Fixes zd#17977

Credit to wolfSentry for the fix

Customer to confirm fix, please do not merge before then

Testing

Built in tests with --enable-dtlscid --enable-tls13 --enable-tlsx --enable-dtls13 --enable-dtls which should cover all cases

Checklist

• [ ] added tests
• [ ] updated/added doxygen
• [ ] updated appropriate READMEs
• [ ] Updated manual and documentation