Introduce CPPSORT_DEPRECATED, disable it in the test suite

This is the first step to needed to deprecate and remove components from
the library (see issues #167 and #168).

include/cpp-sort/detail/config.h

@@ -1,7 +1,7 @@
 /*
  * The MIT License (MIT)
  *
- * Copyright (c) 2016-2019 Morwenn
+ * Copyright (c) 2016-2020 Morwenn
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -96,4 +96,19 @@
 #   endif
 #endif
 
+////////////////////////////////////////////////////////////
+// CPPSORT_DEPRECATED
+
+// [[deprecated]] is available since C++14, hence why this
+// macro is not in attributes.h: we hide it behind a macro
+// to make sure that it can be silenced
+
+#ifndef CPPSORT_DEPRECATED
+#   ifndef CPPSORT_DISABLE_DEPRECATION_WARNINGS
+#       define CPPSORT_DEPRECATED(message) [[deprecated(message)]]
+#   else
+#       define CPPSORT_DEPRECATED(message)
+#   endif
+#endif
+
 #endif // CPPSORT_DETAIL_CONFIG_H_

testsuite/CMakeLists.txt

@@ -36,6 +36,8 @@ macro(configure_tests target)
         CATCH_CONFIG_DISABLE_MATCHERS
         # Enable assertions for more thorough tests
         CPPSORT_ENABLE_ASSERTIONS
+        # We test deprecated code but we don't want it to warn
+        CPPSORT_DISABLE_DEPRECATION_WARNINGS
     )
 
     # More warnings and settings