[RFC PATCH] add xxhash

This patch set replace md4 and murmur2 in ccache with xxhash.
Murmur2 replaced with xxhash32
md4 with xxhash64 - tail of hash just zeroed for now;

1. All test successful passed
2. Added 4 useful things:
HSTATE macro - for replace hash structure in all files
HSIZE - if one want change hash size (working buggy for now)
hash_get_len(HSTATE_T) - for get totalN from different hash structures (xxhash use total_len, md4 use totalN as names for same data)
hash_simple - abstract for 32bit hashes in whole ccache

3. performance tests:
./perf.py --ccache ccache gcc -DHAVE_CONFIG_H -DSYSCONFDIR=/usr/local/etc -I. -I. -MD -MP -MF .deps/ccache.d -g -O2 -Wall -W -Werror ./ccache.c
Compilation command: gcc -DHAVE_CONFIG_H -DSYSCONFDIR=/usr/local/etc -I. -I. -MD -MP -MF .deps/ccache.d -g -O2 -Wall -W -Werror ./ccache.c -c -o ./ccache.o
Compilercheck: mtime
Compression: off
Hardlink: off
Nostats: off

[$] make perf #xxhash
Without ccache: 17.02 s (100.00 %) ( 1.00 x)
With ccache, preprocessor mode, cache miss: 17.90 s (105.18 %) ( 0.95 x)
With ccache, preprocessor mode, cache hit: 1.08 s ( 6.33 %) (15.79 x)
With ccache, direct mode, cache miss: 17.77 s (104.36 %) ( 0.96 x)
With ccache, direct mode, cache hit: 0.08 s ( 0.47 %) (211.13 x)

[$] make perf #md4 with murmur2
Without ccache: 17.44 s (100.00 %) ( 1.00 x)
With ccache, preprocessor mode, cache miss: 18.19 s (104.30 %) ( 0.96 x)
With ccache, preprocessor mode, cache hit: 1.10 s ( 6.31 %) (15.85 x)
With ccache, direct mode, cache miss: 18.14 s (104.01 %) ( 0.96 x)
With ccache, direct mode, cache hit: 0.11 s ( 0.61 %) (164.25 x)


Please review code style

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) and break total_len logic (ccache store len of input data)

2. Zeroing not used hash piece, now implemented and working good
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.