add cmake lz4 support

[LeeYoung624]

Add some instructions in build/cmake/README.md for guys not familiar with cmake. Show them how to build with options
Yes, I saw there is a cmake support, but it can be improved using a CMakeLists.txt file and have a flag for each thing we want to build or not.

Add lz4 format support for zstd program, with a new cmake module FindLibLZ4.cmake added to find lz4 library.
This has been test on:
suse12sp2 cmake 3.12.1 lz4 1.8.3
windows10 cmake 3.12.1 lz4 1.8.3
ubuntu12.04 cmake 3.0.2 lz4 1.8.3
centos7.4 cmake 2.8.12 lz4 1.8.3
redhat7.2 cmake 3.0.2 lz4 1.8.3
with tests like:
lz4 compressed file->zstd decompress file->diff files
zstd compressed file->lz4 decompress file->diff files
Issue#715

I rebased my last pull request and restart a new pull request again

[Cyan4973]

Thanks for the patch @LeeYoung624 ,
it looks clear and is well documented !