WCDH: Make the header -Wundef safe for the C language.

The __STDC_VERSION__ macro may be defined or not depending on the
implementation dialect of C.  Test that it is defined before testing
its value.

The CXX tests do not need such a change because they define __cplusplus
in all dialects.
Merge topic 'C-features-Wundef' into next

d0af0fae WCDH: Make the header -Wundef safe for the C language.