OK it just works. See bellow.

Changed some stuff in levels.lua.
Added levels.c to .gitignore.
New player function `reset_old_pos`.

.gitignore

@@ -1 +1,2 @@
 build*/**
+src/levels.c

levels.lua

@@ -17,8 +17,8 @@ levels = {[[
 -0.........................-
 -0.........................-
 -0.........................-
--0........................00
--0........................0-
+-0.......................000
+-0.......................00-
 -00000000000000000000000000-
 ----------------------------
 ]],

src/levels.c

@@ -1,13 +0,0 @@
-#include "levels.h"
-#include <gint/std/string.h>
-void set_level(int level_id, char *level) {
-  switch (level_id)
-  {
-    case 0:
-      memcpy(level, "-----------------------------00000000000000000000000000--0........................0--0........................0--0........................0--0........................0--0........................0--0........................0--0........................0--0........................00-0.........................--0.........................--0.........................--0........................00-0........................0--00000000000000000000000000-----------------------------", 476);
-    break;
-    case 1:
-      memcpy(level, "-----------------------------00000000000000000000000000--00000000000000000000000000--000000000000000000000000000-00000000000...............--0000000000................--0000000000....0000000000000-0000000000.....00000000000--00000000..........00000000-000000.............00000000--..........-.00000000000000--..........0000000000000000--.......0000000000000000000-000000000000000000000000000--00000000000000000000000000--00000000000000000000000000-----------------------------", 476);
-    break;
-  }
-}

src/main.c

@@ -114,21 +114,23 @@ int main(void)
       player_y = start_y;
     }
     //borders collision
-    else if (player_x <= 2)
+    if (player_x <= 8)
     {
-      level_id--;
-      player_x = 229;
+      level_id -= 1;
+      player_x = 201;
       set_start_pos(&start_x, &start_y, player_x, player_y);
       set_level(level_id, level);
       draw_level(level);
+      reset_old_pos(&old_x, &old_y);
     }
-    else if (player_x >= 222)
+    else if (player_x >= 208)
     {
-      level_id++;
+      level_id += 1;
       player_x = 9;
       set_start_pos(&start_x, &start_y, player_x, player_y);
       set_level(level_id, level);
       draw_level(level);
+      reset_old_pos(&old_x, &old_y);
     }
 		//exit
 		if (keydown(KEY_EXIT)) return 0;

src/player.c

@@ -22,3 +22,9 @@ void set_start_pos(int *start_x, int *start_y, int x, int y)
   *start_x = x;
   *start_y = y;
 }
+
+void reset_old_pos(int *old_x, int *old_y)
+{
+	*old_x = -8;
+	*old_y = -8;
+}

src/player.h

@@ -1,2 +1,3 @@
 void jump_test(char *jump_pressed, char *jump_buffer, unsigned int *jump_hold);
 void set_start_pos(int *start_x, int *start_y, int x, int y);
+void reset_old_pos(int *old_x, int *old_y);