Use Jenkins hash.

Use Jenkins hash.

This is about twice as fast as the current hash, and removes the
need for double hashing (improving locality of reference).  The
hash function is based on Bob Jenkins' design, slightly adapted
wherever Make needs to hash NUL-terminated strings.  The old hash
function is kept for case-insensitive hashing.

This saves 8.5% on QEMU's no-op build (from 12.87s to 11.78s).

* configure.ac: Check endianness.
* hash.c (rol32, jhash_mix, jhash_final, JHASH_INITVAL,
sum_get_unaligned_32, jhash): New.
* hash.h (STRING_HASH_1, STRING_N_HASH_1): Use jhash.
(STRING_HASH_2, STRING_N_HASH_2): Return a dummy value.
(STRING_N_COMPARE, return_STRING_N_COMPARE): Prefer memcmp to strncmp.