allow atf to be used in a C-only environment

[emaste]

Discussed in https://reviews.freebsd.org/D7597. It looks like FreeBSD will disable tests when building without C++, as some ATF components require a C++ compiler. I don't believe this is a high priority as FreeBSD does build a C++ toolchain by default, but if it's straightforward to migrate (a minimal set of) ATF components to C we could enable/disable tests on a more fine-grained basis.

[jmmv]

Ah, yikes. I remember now why this is not trivial: atf-sh also depends on atf-check and, while the former is trivial to rewrite in C, the latter is not.

My real plan would be to migrate the atf-sh tests to shtk, which has a much more robust testing library for shell tests and is purely written in shell... but that's much more effort than I can currently take on :-/

[krytarowski]

This is must have in NetBSD for me. C++ dependency is a higher cost than executing almost all of the tests in NetBSD. This matters with external toolchain, sanitized userland, ports with incomplete C++ support (not everybody needs C++, there are/were LLVM backends without C++ support).