This is cast to a function-pointer:

((type) value)

Where type is void (*)(int) which is pointer to function accepting one int argument and returning void, which is actually is a signature of a signal handler:

typedef void (*sighandler_t)(int);

You may decode such types with cdecl tool or web-site: http://cdecl.org/

This is a cast of -1 into the function pointer which is expected as the type of SIG_ERR. Using -1 directly does not work at situations where the compiler needs the correct type.