Thank you all for the answers, I've learned some things. First of all, about the old naming standard it seems that the package itself is old and not very maintained, the homepage seems to be offline. Looking around the code I found the typedefs:
#if (__sun && __SVR4)
/* libnet should be using the standard type names, but in the short term
* define our non-standard type names in terms of the standard names.
*/
#include <inttypes.h>
typedef uint8_t u_int8_t;
typedef uint16_t u_int16_t;
typedef uint32_t u_int32_t;
typedef uint64_t u_int64_t;
#endif
The #if was originaly like this:
#if (__sun__ $$ svr4)
Both of the macros are deffined differently in the system. After the change, it worked fine.
Thanks again!