utils: Add memrchr(3) replacement for platforms that don't support it

For instance, on Mac OS X memrchr(3) is not provided by the C library.

src/libstrongswan/utils/utils.h

@@ -464,6 +464,20 @@ static inline void memwipe(void *ptr, size_t n)
  */
 void *memstr(const void *haystack, const char *needle, size_t n);
 
+/**
+ * Replacement for memrchr(3) if it is not provided by the C library.
+ *
+ * @param s		start of the memory area to search
+ * @param c		character to search
+ * @param n		length of memory area to search
+ * @return		pointer to the found character or NULL
+ */
+void *utils_memrchr(const void *s, int c, size_t n);
+
+#ifndef HAVE_MEMRCHR
+#define memrchr(s,c,n) utils_memrchr(s,c,n)
+#endif
+
 /**
  * Translates the characters in the given string, searching for characters
  * in 'from' and mapping them to characters in 'to'.