Clean up plugin path handling

Clean up plugin path handling

Drop --with-plugindir, instead use an environment variable PLUGINDIR
to specify the plugin directory.

This puts a define into include/openvpn-plugin.h.in which has the plugin
directory.

The configure script does not know about the final plugin path. Thus we
have to make Make generate the final header file for us.

As the path is always available remove the compile time condition (and
dead code) from src/openvpn/plugin.c.

v2: The configure script can not evaluate the final $libdir path. So
    use make to create a header file on the containing the final path.

v3: Fix whitespace errors and gitignore location.

v4: No extra header file, generate src/openvpn/plugin.h on the fly.
    Remove condition and dead code.

v5: Move the define to include/openvpn-plugin.h.in and let make generate
    the final header file.

Signed-off-by: Christian Hesse <mail@eworm.de>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20170125201947.17197-1-list@eworm.de>
URL: http://www.mail-archive.com/search?l=mid&q=20170125201947.17197-1-list@eworm.de
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 4590c3831d0400096fab08aa1ed7f909da870ced)