plugin: Remove GNUism in openvpn-plugin.h generation

plugin: Remove GNUism in openvpn-plugin.h generation

The plugin path handling cleanup (4590c383) introduced GNUism and broke
builds on system not using GNU Make (like *BSD).

Revert back to let configure generate the header file. Instead let make
add an extra CFLAG that defines PLUGIN_LIBDIR.

Signed-off-by: Christian Hesse <mail@eworm.de>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20170127084927.21040-1-list@eworm.de>
URL: http://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13966.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 631812fe29c69d0034628ab8321cb4016cb4fc2d)