Implement a simple xorshift 32bit random number generator

Implement a simple xorshift 32bit random number generator

Avoid relying on the system random number generator for our random
shuffle, so that the same seed gives the same results on all systems.
This generator doesn't need to be amazing, just pretty good, so don't
bother with xorshift* or xorshift+, etc.

* src/makeint.h: Declare make_seed() and make_rand().
* src/misc.c (make_seed): Set the seed value for the RNG.
(make_rand): Return the next random number.  If the seed was not set
initialize it first.
* src/shuffle.c (shuffle_set_mode): If we don't get a seed from the
user just leave it unset (0).
(shuffle_deps_recursive): Use make_seed() not srand().
(random_shuffle_array): Use make_rand() not rand().