using CPP with configure

[Patrick Venture]

You might have seen a patchset from me setting the language to C++ in
your configure script.

Apparently the default language in the autotools is C. Why does this
matter?  It tries to compile headers and other pieces in C to
determine if they're available.

How this is fixed, by setting the language to cpp before the header checks.

https://gerrit.openbmc-project.xyz/q/topic:%22build_cpp%22+(status:open%20OR%20status:merged)

For new repos, please set the language accordingly.

Patrick