[xml] cleanup on randomization - inconsistent in master

[Roumen Petrov <bugtrack roumenpetrov info>]

Hi All,

One of recent updates swicth from rand to rand_r but it seems to me 
switch is incomplete:
- dict.c - #ifdef HAVE_RAND_R
- libxml.h -   +#if defined(HAVE_RAND)
- configure check for rand function but not for rand_r

Please to update configure script and libxml.h header to be consistent.

Post 2.8.0 could implement fail back solution to rand as not all 
platforms provide rand_r.

Roumen