Add pg_strnlen() a portable implementation of strlen.

As the OS version is likely going to be more optimized, fall back to
it if available, as detected by configure.

src/include/common/string.h

@@ -12,4 +12,19 @@
 
 extern bool pg_str_endswith(const char *str, const char *end);
 
+/*
+ * Portable version of posix' strnlen.
+ *
+ * Returns the number of characters before a null-byte in the string pointed
+ * to by str, unless there's no null-byte before maxlen. In the latter case
+ * maxlen is returned.
+ *
+ * Use the system strnlen if provided, it's likely to be faster.
+ */
+#ifdef HAVE_STRNLEN
+#define pg_strnlen(str, maxlen) strnlen(str, maxlen)
+#else
+extern size_t pg_strnlen(const char *str, size_t maxlen);
+#endif
+
 #endif							/* COMMON_STRING_H */