Fix: remove .so on uninstall and set +x the .so on install

For some reasons (unknown), autoconf sets the executable bit to the
installed .so when using the plugin_LTLIBRARIES statement. This puts
back the chmod in the install hook and adds an uninstall hook to remove
the .so that is simply *not* removed during the normal make uninstall.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>
Fix: remove gmodule useless linked library flags

Only the CFLAGS are needed for irssi headers to make sense.

Ref #31

Reported-by: Antoine Beaupré <anarcat@debian.org>
Signed-off-by: David Goulet <dgoulet@ev0ke.net>