Tests: Fix name of cache variable.
Help: Fix order of help entries.
Project: Remove extern from static string in feature tests.
Project: Use nullary form of main for compile feature tests.
Project: Fix exit-on-error with compile feature tests.
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

3791c3b1 Features: Extend concept to C language.
1461ad4c Project: Fix exit-on-error with compile feature tests.
402e6af7 Project: Use nullary form of main for compile feature tests.
261bfc64 Project: Remove extern from static string in feature tests.
850daa34 Help: Fix order of help entries.
da54424e Tests: Fix name of cache variable.