Explain <isc/strerr.h> a little more

I tried to delete it, but it isn't as crufty as it appeared to be.

lib/isc/include/isc/strerr.h

@@ -26,4 +26,8 @@
 #if defined(strerror_r)
 #undef strerror_r
 #endif /* if defined(strerror_r) */
+
+/*
+ * Ensure we use a consistent implementation of strerror_r()
+ */
 #define strerror_r isc_string_strerror_r

lib/isc/string.c

@@ -55,6 +55,14 @@
 
 #include <isc/string.h> /* IWYU pragma: keep */
 
+/*
+ * We undef _GNU_SOURCE above to get the POSIX strerror_r()
+ */
+int
+isc_string_strerror_r(int errnum, char *buf, size_t buflen) {
+	return (strerror_r(errnum, buf, buflen));
+}
+
 #if !defined(HAVE_STRLCPY)
 size_t
 strlcpy(char *dst, const char *src, size_t size) {
@@ -136,8 +144,3 @@ strnstr(const char *s, const char *find, size_t slen) {
 	return ((char *)s);
 }
 #endif
-
-int
-isc_string_strerror_r(int errnum, char *buf, size_t buflen) {
-	return (strerror_r(errnum, buf, buflen));
-}