[RFC PATCH] add xxhash

This is patch set just for review (it's safe for merge and use it)

Now it only add xxhash to sources and replace murmur hash with xxhash32
murmur replacement working without issues and i use it on my system.

Please review code style

I have a plan for replace md4 algorithm with xxhash. xxhash faster, but only theoretical.
Also xxhash, not have 128bit version. How it can be fixed:
1. Hack https://github.com/Cyan4973/xxHash/pull/13
Use 2 independent 64bit functions for hash generating (safe only for static block sizes)

2. Zeroing not used hash piece
https://github.com/markfasheh/duperemove/commit/fc6fec73c667c9d77645154be74fba54258ee072
https://github.com/markfasheh/duperemove/pull/35

3. Move ccache to 64bit hash logic
This is strange idea
I think what ccache use 128 bit logic only because md4 return 128bit hash -> save move to xxhash 64
But i want mention to https://github.com/Cyan4973/xxHash/pull/13 , i believe in Cyan4973, that he can release 128bit version.