Add LZ4LIB_API

Add LZ4LIB_API

It's based on Zstandard's ZSTDLIB_API.

See also: https://github.com/Cyan4973/lz4/issues/216#issuecomment-226245432

Deprecated functions aren't LZ4LIB_API targets. Because we don't need to
export deprecated functions from now.

There are same LZ4LIB_API definitions in each header files instead of
including a common header file because LZ4_DEPRECATED is defined so.