Implement LZ4 compression.

Implement LZ4 compression, similar to the existing snappy / push-peer-info
model: a LZ4 capable client will send IV_LZ4=1 to the server, and the
algorithm is selected by pushing "compress lz4" back.

LZ4 does not compress as well as LZO or Snappy, but needs far less CPU
and is much faster, thus better suited for mobile devices.  See
https://code.google.com/p/lz4/ for more details.

LZ4 include and library path can be specified by specifying LZ4_LIBS=...
and LZ4_CFLAGS=... on the configure command line.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1388613479-22377-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8153
Provide LZ4 sources in src/compat/ and use if no system lz4 library found.

Bundle lz4.c and lz4.h from http://code.google.com/p/lz4/ (r109) as
src/compat/compat-lz4.[ch], and use that (via #define NEED_COMPAT_LZ4)
if autoconf cannot find lz4.h or -llz4 in the system.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1388613479-22377-2-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8154