Second attempt to fix the use of aligned_alloc with BSD

libs/spandsp/src/alloc.c

@@ -36,10 +36,11 @@
 #if !defined(__USE_ISOC11)
 #define __USE_ISOC11
 #endif
-#include <stdlib.h>
 #if defined(__ISO_C_VISIBLE)  &&  __ISO_C_VISIBLE < 2011
+#undef __ISO_C_VISIBLE
 #define __ISO_C_VISIBLE 2011
 #endif
+#include <stdlib.h>
 #if defined(HAVE_MALLOC_H)
 #include <malloc.h>
 #endif