Quick answer: change int testlib() to int testlib(void) to specify that the function takes no arguments.
A prototype is by definition a function declaration that specifies the type(s) of the function's argument(s).
A non-prototype function declaration like
int foo();
is an old-style declaration that does not specify the number or types of arguments. (Prior to the 1989 ANSI C standard, this was the only kind of function declaration available in the language.) You can call such a function with any arbitrary number of arguments, and the compiler isn't required to complain -- but if the call is inconsistent with the definition, your program has undefined behavior.
For a function that takes one or more arguments, you can specify the type of each argument in the declaration:
int bar(int x, double y);
Functions with no arguments are a special case. Logically, empty parentheses would have been a good way to specify that an argument but that syntax was already in use for old-style function declarations, so the ANSI C committee invented a new syntax using the void keyword:
int foo(void); /* foo takes no arguments */
A function definition (which includes code for what the function actually does) also provides a declaration. In your case, you have something similar to:
int testlib()
{
/* code that implements testlib */
}
This provides a non-prototype declaration for testlib. As a definition, this tells the compiler that testlib has no parameters, but as a declaration, it only tells the compiler that testlib takes some unspecified but fixed number and type(s) of arguments.
If you change () to (void) the declaration becomes a prototype.
The advantage of a prototype is that if you accidentally call testlib with one or more arguments, the compiler will diagnose the error.
(C++ has slightly different rules. C++ doesn't have old-style function declarations, and empty parentheses specifically mean that a function takes no arguments. C++ supports the (void) syntax for consistency with C. But unless you specifically need your code to compile both as C and as C++, you should probably use the () in C++ and the (void) syntax in C.)