We can't assume that strerror_r returns char* just because _GNU_SOURCE is defined. We already call the appropriate autoconf test, so let's use its result 
(STRERROR_R_CHAR_P).

Note that in configure, _GNU_SOURCE is always set, but we add a defined guard just in case for futureproofing.

Bug: https://bugs.gentoo.org/869404
--- a/daemons/lvmpolld/lvmpolld-core.c
+++ b/daemons/lvmpolld/lvmpolld-core.c
@@ -52,7 +52,7 @@ static pthread_key_t key;

 static const char *_strerror_r(int errnum, struct lvmpolld_thread_data *data)
 {
-#ifdef _GNU_SOURCE
+#if defined(_GNU_SOURCE) && defined(STRERROR_R_CHAR_P)
        return strerror_r(errnum, data->buf, sizeof(data->buf)); /* never returns NULL */
 #elif (_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)
        return strerror_r(errnum, data->buf, sizeof(data->buf)) ? "" : data->buf;