cmake: Debug build sets DEBUG, properly find check lib

include/ucore/sarena.h

@@ -25,9 +25,9 @@ extern "C" {
 */
 typedef struct UCSArena UCSArena;
 struct UCSArena {
-    char *start; // start of user buffer
-    char *curr;  // next alloc point
-    char *end;   // one past end of user buffer
+    unsigned char *start; // start of user buffer
+    unsigned char *curr;  // next alloc point
+    unsigned char *end;   // one past end of user buffer
 };
 
 /** Initialize a UCSArena with predefined backing array.