Fix compilation issues with GCC and -Wextra

[saghul]

/home/saghul/src/quickjs/examples/../quickjs.h: In function ‘JS_NewCFunctionMagic’:
/home/saghul/src/quickjs/examples/../quickjs.h:919:34: error: cast between incompatible function types from ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, int)’ to ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *)’ [-Werror=cast-function-type]
  919 |     return JS_NewCFunction2(ctx, (JSCFunction *)func, name, length, cproto, magic);
      |                                  ^
/home/saghul/src/quickjs/examples/../quickjs.h:919:34: error: cast between incompatible function types from ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, int)’ to ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *)’ [-Werror=cast-function-type]
  919 |     return JS_NewCFunction2(ctx, (JSCFunction *)func, name, length, cproto, magic);
      |                                  ^
In file included from /home/saghul/src/quickjs/tests/../quickjs-libc.h:30,
                 from /home/saghul/src/quickjs/tests/bjson.c:24:
/home/saghul/src/quickjs/tests/../quickjs.h: In function ‘JS_NewCFunctionMagic’:
/home/saghul/src/quickjs/tests/../quickjs.h:919:34: error: cast between incompatible function types from ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, int)’ to ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *)’ [-Werror=cast-function-type]
  919 |     return JS_NewCFunction2(ctx, (JSCFunction *)func, name, length, cproto, magic);
      |                                  ^
cc1: all warnings being treated as errors
make[3]: *** [CMakeFiles/bjson.dir/build.make:79: CMakeFiles/bjson.dir/tests/bjson.c.o] Error 1
make[3]: Leaving directory '/home/saghul/src/quickjs/build'
make[2]: *** [CMakeFiles/Makefile2:342: CMakeFiles/bjson.dir/all] Error 2
make[2]: *** Waiting for unfinished jobs....
In file included from /home/saghul/src/quickjs/quickjs.c:47:
/home/saghul/src/quickjs/quickjs.h: In function ‘JS_NewCFunctionMagic’:
/home/saghul/src/quickjs/quickjs.h:919:34: error: cast between incompatible function types from ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, int)’ to ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *)’ [-Werror=cast-function-type]
  919 |     return JS_NewCFunction2(ctx, (JSCFunction *)func, name, length, cproto, magic);
      |                                  ^
cc1: all warnings being treated as errors
make[3]: *** [CMakeFiles/fib.dir/build.make:79: CMakeFiles/fib.dir/examples/fib.c.o] Error 1
make[3]: Leaving directory '/home/saghul/src/quickjs/build'
make[2]: *** [CMakeFiles/Makefile2:290: CMakeFiles/fib.dir/all] Error 2
cc1: all warnings being treated as errors
make[3]: *** [CMakeFiles/point.dir/build.make:79: CMakeFiles/point.dir/examples/point.c.o] Error 1
make[3]: Leaving directory '/home/saghul/src/quickjs/build'
make[2]: *** [CMakeFiles/Makefile2:316: CMakeFiles/point.dir/all] Error 2
/home/saghul/src/quickjs/quickjs.c: In function ‘js_append_enumerate’:
/home/saghul/src/quickjs/quickjs.c:13534:40: error: cast between incompatible function types from ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, int)’ to ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *)’ [-Werror=cast-function-type]
13534 |                                        (JSCFunction *)js_create_array_iterator,
      |                                        ^
/home/saghul/src/quickjs/quickjs.c:13547:37: error: cast between incompatible function types from ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, BOOL *, int)’ {aka ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, int *, int)’} to ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *)’ [-Werror=cast-function-type]
13547 |     &&  JS_IsCFunction(ctx, method, (JSCFunction *)js_array_iterator_next, 0)
      |                                     ^
/home/saghul/src/quickjs/quickjs.c: In function ‘add_import’:
/home/saghul/src/quickjs/quickjs.c:26366:55: error: implicit conversion from ‘enum <anonymous>’ to ‘JSVarKindEnum’ [-Werror=enum-conversion]
26366 |                               local_name, TRUE, TRUE, FALSE);
      |                                                       ^~~~~
/home/saghul/src/quickjs/quickjs.c: In function ‘add_module_variables’:
/home/saghul/src/quickjs/quickjs.c:30165:45: error: implicit conversion from ‘enum <anonymous>’ to ‘JSVarKindEnum’ [-Werror=enum-conversion]
30165 |                             hf->is_lexical, FALSE) < 0)
      |                                             ^~~~~
/home/saghul/src/quickjs/quickjs.c: In function ‘JS_AddIntrinsicPromise’:
/home/saghul/src/quickjs/quickjs.c:45610:34: error: cast between incompatible function types from ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, int)’ to ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *)’ [-Werror=cast-function-type]
45610 |     obj1 = JS_NewCFunction3(ctx, (JSCFunction *)js_function_constructor,
      |                                  ^
/home/saghul/src/quickjs/quickjs.c:45646:34: error: cast between incompatible function types from ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, int)’ to ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *)’ [-Werror=cast-function-type]
45646 |     obj1 = JS_NewCFunction3(ctx, (JSCFunction *)js_function_constructor,
      |                                  ^
/home/saghul/src/quickjs/quickjs.c: In function ‘JS_AddIntrinsicBaseObjects’:
/home/saghul/src/quickjs/quickjs.c:47187:42: error: cast between incompatible function types from ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, int)’ to ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *)’ [-Werror=cast-function-type]
47187 |         func_obj = JS_NewCFunction3(ctx, (JSCFunction *)js_error_constructor,
      |                                          ^
/home/saghul/src/quickjs/quickjs.c: In function ‘JS_AddIntrinsicTypedArrays’:
/home/saghul/src/quickjs/quickjs.c:50359:42: error: cast between incompatible function types from ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *, int)’ to ‘JSValue (*)(JSContext *, JSValue,  int,  JSValue *)’ [-Werror=cast-function-type]
50359 |         func_obj = JS_NewCFunction3(ctx, (JSCFunction *)js_typed_array_constructor,
      |                                          ^
/home/saghul/src/quickjs/quickjs.c: In function ‘js_json_check’:
/home/saghul/src/quickjs/quickjs.c:41843:12: error: this statement may fall through [-Werror=implicit-fallthrough=]
41843 |         if (JS_IsFunction(ctx, val))
      |            ^
/home/saghul/src/quickjs/quickjs.c:41845:5: note: here
41845 |     case JS_TAG_STRING:
      |     ^~~~
cc1: all warnings being treated as errors