Since the musl libc constellation results in unistd.h being
included, which defines the getopt() function but not the
getopt globals, define them here as well.

--- src/dar_suite/my_getopt_long.h	2015-10-17 10:42:50.000000000 +0200
+++ src/dar_suite/my_getopt_long.h	2015-10-18 13:13:32.631006858 +0200
@@ -27,6 +27,12 @@
 #ifndef MY_GETOPT_LONG_H
 #define MY_GETOPT_LONG_H
 
+#if !defined(__GLIBC__)
+// declare externs which would be defined when including getopt.h
+extern char *optarg;
+extern int optind, opterr, optopt, optreset;
+#endif
+
 // getopt may be declated in <unistd.h> on systems like FreeBSD.
 // if you want to use libgnugetopt you need to include <getopt.h>
 // on this system. Thus a conflict appear because the getopt is