lz4-sys: link against system liblz4 if possible

[esmil]

This brings lz4-sys in line with lzma-sys and libz-sys.

[benesch]

@jheyens what would it take to get this merged as well? Similar justification as #25; i.e., most sys crates provide a way to dynamically link against the library installed upon the system. Merging this as is would bring it in line with http://github.com/alexcrichton/xz2-rs, but would be a breaking change, since it changes the default from statically linking the vendored copy to dynamically linking the system copy.

[jheyens]

Since this is a breaking change, I want this to be merged in the upcoming version 2.0.

Version 2 will add breaking changes by modifying the block module API to not force allocation upon the user and add LZ4's stream mode, which is still missing.

Additionally, I want to mirror this API for stream and frame mode to allow users to choose the one best matching their use-case.

I (locally) started working on adding stream mode earlier this year, but I stopped working on it in favor of university stuff.

I would like this to be finished at the end of this year, though.