core/lib: compiler_hints guard NORETURN and PURE

core/lib/include/compiler_hints.h

@@ -30,11 +30,13 @@ extern "C" {
  * @brief The *NORETURN* keyword tells the compiler to assume that the function
  *        cannot return.
  */
+#ifndef NORETURN
 #ifdef __GNUC__
 #define NORETURN  __attribute__((noreturn))
 #else
 #define NORETURN
 #endif
+#endif
 
 /**
  * @def PURE
@@ -43,11 +45,13 @@ extern "C" {
  *        function can be subject to common subexpression elimination and loop
  *        optimization just as an arithmetic operator would be.
  */
+#ifndef PURE
 #ifdef __GNUC__
 #define PURE  __attribute__((pure))
 #else
 #define PURE
 #endif
+#endif
 
 /**
  * @def       UNREACHABLE()