lz4: Fix broken builds when pkg-config is not present but system library is

lz4: Fix broken builds when pkg-config is not present but system library is

In commit f91e4863bc1382 we fixed an issue where LZ4_LIBS could be
overwritten in some situations.  But on systems where lz4 is installed on
the system but is lacking pkg-config information, the linker will not know
about the lz4 library when completing the build.

This fixes the issue by explicitly setting LZ4_LIBS to contain -llz4
if pkg-config test was run and failed verifying the installed lz4 version
number.  This also ensures that LZ4_LIBS will not be overwritten if it
has been provided on the ./configure command line.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20171002190732.12531-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15549.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>