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

7b1cdd86 WCDH: Make the header -Wundef safe for the C language.
Add missing setting.
Merge topic 'WCDH-multi-file' into next

7b67cdab Add missing setting.
WCDH: Make it possible to generate multiple files.

Extend the write_compiler_detection_header interface to allow
specifying a location for supplementary files, and getting the
list of resulting files as a variable.
Merge topic 'WCDH-multi-file' into next

4cf5179c WCDH: Make it possible to generate multiple files.