Convert app to c++

libs/lib3/inc/lib3.h

@@ -1,6 +1,10 @@
 #pragma once
 #include <stdio.h> // Standard I/O header is needed here because printf is used within this header-only function.
 
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 // Declared as static to prevent multiple definition errors.
 // Since this is a header-only function, it will be included and compiled
 // into every translation unit that includes lib3.h. 'static' limits its
@@ -9,3 +13,7 @@ static void lib3_function()
 {
   printf("This is a function from Header-Only Library 3.\n");
 }
+
+#if defined(__cplusplus)
+}
+#endif