lz4: Move towards a newer LZ4 API

lz4: Move towards a newer LZ4 API

We are using a deprecated function, LZ4_compress_limitedOutput(), which
will be removed with time.  The correct function to use is
LZ4_compress_default().  Both function takes the same number of
arguments and data types, so the change is minimal.

This patch will also enforce the system LZ4 library to be at least v1.7.1.
If the system library is not found or it is older, it will be build using
the bundled LZ4 library.  The version number requirement is based on the
LZ4 version we ship.

The changes in configure.ac for the version check is modelled around the
same approach we use for OpenSSL.  Plus it does a few minor reformats and
improvements to comply with more recommend autoconf coding style.

This patch is a result of the discussions in this mail thread:
https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14135.html

Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20170907172004.22534-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15396.html
Signed-off-by: David Sommerseth <davids@openvpn.net>