Fix test.
Merge topic 'compile-features-C-language' into next

548f1953 Fix test.
Features: Extend concept to C language.

Add properties and variables corresponding to CXX equivalents.

Add features for c_function_prototypes (C90), c_restrict (C99),
c_variadic_macros (C99) and c_static_assert (C11). This can be
extended later.
Merge topic 'compile-features-C-language' into next

64db773b Features: Extend concept to C language.