build(CMake): Require C99 mode

Specify language requirement as a [compile-feature] and force compiler
extensions off ensuring portability problems are detected early on.
Note that we do not use the `C_STANDARD` property, because it doesn't
propagate to dependent targets and would prohibit users from compiling
their code base with consistent flags / language configuations if they
were to target a newer C standard. Similarly we do not configure
`C_STANDARD_REQUIRED` as [compile-features] do not interact with
it--they are enforced regardless.

[compile-feature]: https://cmake.org/cmake/help/latest/manual/cmake-compile-features.7.html#compile-feature-requirements

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