Define _POSIX_C_SOURCE if undefined

config.c uses _POSIX_C_SOURCE which is defined in features.h when
glibc/uclibc is used, but isn't defined when musl is used.
So provide a reasonable default.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.com>

config.c

@@ -63,6 +63,9 @@
  * but may not wrap over lines
  *
  */
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
 
 #ifndef CONFFILE
 #define CONFFILE "/etc/mdadm.conf"