utils.h: use <stdint.h> (if available) instead of reinventing it.

utils.h: use <stdint.h> (if available) instead of reinventing it.

The argument of golden_ratio_hash() is as an integer of the same size as
a void* pointer. Unfortunately ISO C++ 98 does not provide suitable typedef.
Hence

* use <stdint.h> if available and define p_int to uintptr_t. Note: AC_PROG_CC
  already checks for this header, so no extra checks are necessary.
* as a fallback define p_int to be unsigned long, this works on most systems
  I know of (the only exception is woe64).

While at it, stop including "config.h" unconditionally.