src: polyfill the 'include' keyword for Visual Studio 2013

src/queue.c

@@ -2,6 +2,7 @@
 #include <stdlib.h>
 
 #include "queue.h"
+#include "util.h"
 
 void queue_init(queue_t* queue) {
   queue_node_init(&queue->head);

src/tree.c

@@ -3,6 +3,7 @@
 #include "error.h"
 #include "rb.h"
 #include "tree.h"
+#include "util.h"
 
 static inline int _tree_compare(tree_node_t* a, tree_node_t* b) {
   if (a->key < b->key)

src/util.h

@@ -20,6 +20,11 @@ typedef intptr_t ssize_t;
 
 #define unused(v) ((void) (v))
 
+#if defined(_MSC_VER) && _MSC_VER < 1900
+/* Polyfill `inline` for msvc 12 (Visual Studio 2013) */
+#define inline __inline
+#endif
+
 #ifdef __clang__
 /* Polyfill static_assert() because clang doesn't support it. */
 #define static_assert(condition, message) typedef __attribute__( \