The musl libc strerror_r(3) semantics are different from
GNU libc's if _GNU_SOURCE is defined.

--- a/src/api.c	2014-01-13 15:05:56.000000000 +0100
+++ b/src/api.c	2015-10-25 14:09:18.690232596 +0100
@@ -3332,8 +3332,16 @@
 
 const char *cgroup_strerror(int code)
 {
+#if defined(__GLIBC__)
 	if (code == ECGOTHER)
 		return strerror_r(cgroup_get_last_errno(), errtext, MAXLEN);
+#else
+	if (code == ECGOTHER) {
+		if (0 == strerror_r(cgroup_get_last_errno(), errtext, MAXLEN))
+			return errtext;
+		return "strerror_r() failed";
+	}
+#endif
 
 	return cgroup_strerror_codes[code % ECGROUPNOTCOMPILED];
 }