Avoid warnings by defining prototypes for crc32 functions

crc32.c:14:6: warning: no previous prototype for function 'make_crc_table' [-Wmissing-prototypes]
void make_crc_table(void)
     ^
crc32.c:46:15: warning: no previous prototype for function 'update_crc' [-Wmissing-prototypes]
unsigned long update_crc(unsigned long crc,
              ^
crc32.c:61:15: warning: no previous prototype for function 'crc32' [-Wmissing-prototypes]
unsigned long crc32(unsigned char *buf, int len)
              ^
3 warnings generated.
Ignore generated files
static, so that the compiler tell us what is not being used and that it won't share the symbols to other code avoiding possible clashes there
Merge pull request #2 from massar/master

A few more warning fixes (prototype defines) + makes everything static