Allow for overriding feature test macros

liberty.c

@@ -16,8 +16,13 @@
  *
  */
 
-#define _POSIX_C_SOURCE 199309L
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200112L
+#endif
+
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 600
+#endif
 
 #include <stdio.h>
 #include <stddef.h>
@@ -2450,7 +2455,7 @@ write_queue_processed (struct write_queue *self, size_t len)
 }
 
 static bool
-write_queue_is_empty (struct write_queue *self)
+write_queue_is_empty (const struct write_queue *self)
 {
 	return self->head == NULL;
 }
@@ -2545,7 +2550,7 @@ msg_unpacker_make (const void *data, size_t len)
 }
 
 static size_t
-msg_unpacker_get_available (struct msg_unpacker *self)
+msg_unpacker_get_available (const struct msg_unpacker *self)
 {
 	return self->len - self->offset;
 }