Sanitise unsafe functions, GNUC annotations

This is an attempt to sanitise unsafe functions in libknot API which allow easy out-of-bound reads or undefined return value. The other part is annotating most used functions with appropriate attributes.

src/libknot/internal/macros.h

@@ -30,6 +30,21 @@
#define _public_ __attribute__((visibility("default")))
#define _hidden_ __attribute__((visibility("hidden")))
+/*! \brief GNU C function attributes. */
+#if __GNUC__ >= 3
+#define _pure_      __attribute__ ((pure))
+#define _const_     __attribute__ ((const))
+#define _noreturn_  __attribute__ ((noreturn))
+#define _malloc_    __attribute__ ((malloc))
+#define _mustcheck_ __attribute__ ((warn_unused_result))
+#else
+#define _pure_
+#define _const_
+#define _noreturn_
+#define _malloc_
+#define _mustcheck_
+#endif
+
/*! \brief Eliminate compiler warning with unused parameters. */
#define UNUSED(param) (void)(param)