Add a c_standard configuration attribute

We have a `cxx_standard` attribute for setting the c++ standard, we should also provide an analogous configuration for the C standard.

Start by locating the existing cxx_standard configuration attribute and any tests or documentation covering it; the issue does not name a file or test. Add the analogous c_standard configuration, then verify that it controls the C language standard in the same way and that relevant tests pass.

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