and allow override for binary distributions.  Extend mysql_config
to print compiled-in plugin location for third-party plugins to
use.  Resolves bug#31736.


libmysqld/Makefile.am:
  Use pkgplugindir.
plugin/daemon_example/Makefile.am:
  Use pkgplugindir.
plugin/fulltext/Makefile.am:
  Use pkgplugindir.
scripts/Makefile.am:
  Add pkgplugindir.
scripts/make_binary_distribution.sh:
  Add pkgplugindir.
scripts/mysql_config.sh:
  Add pkgplugindir.
sql/Makefile.am:
  Use pkgplugindir.
sql/mysqld.cc:
  Use PLUGINDIR, derived from pkgplugindir, instead of
  LIBDIR for plugins.
sql/unireg.h:
  Use PLUGINDIR instead of LIBDIR, and define to be the
  default setting of pkgplugindir.
storage/innobase/Makefile.am:
  Use pkgplugindir.
storage/archive/Makefile.am:
  Use pkgplugindir.
storage/blackhole/Makefile.am:
  Use pkgplugindir.
storage/example/Makefile.am:
  Use pkgplugindir.
storage/federated/Makefile.am:
  Use pkgplugindir.