configure: display configured OS
lib: use uint8_t instead of u_int8_t

`uint8_t` is part of C99 and comes from `stdint.h`. `u_int8_t` works
on many OS but not on some. There are many other occurrences but we
fix those here because this is the only public header.
configure: use `AC_PROG_EGREP` to find a suitable `grep -E`

On some systems, this is `ggrep -E`.