There are some recursive targets that automake generates which reference
DIST_SUBDIRS.  It's critical, then, for such subdirs to exist even if they
won't be built as part of SUBDIRS.

During a VPATH build, it is the configure script which creates the subdirs
(when it processes the AC_CONFIG_FILES() for each subdir's Makefile).  If
autoconf doesn't create a subdir's Makefile, then the recursive make will fail
when it is unable to cd into that subdir.

This isn't a problem in non-VPATH builds, because the subdirs are all present
in the source tarball.  So the problem only shows up during 'make distcheck',
which does a VPATH build.

The fix is to look, when configure is being created by autoconf, for any
plugin subdirectories.  These are the dynamic subdirectories which need to be
handled specially.  It's enough to tell autoconf to generate a Makefile for
any Makefile.am found in the plugin directory - all plugin subdirectories
using automake (i.e., listed in the plugin's DIST_SUBDIRS) will have a
Makefile.am.

This is done by calling 'find'.  This means that 'find' must be in the PATH on
the host that is running autoconf.  'find' is NOT needed when calling
configure, so it is not an additional dependency for the user.

Finally, ha_ndbcluster.m4 had called AC_CONFIG_FILES() on all those subdir
Makefiles, but only when the plugin was actually being built.  So it didn't
work in the case that NDB was not being built.  All of those Makefiles have to
be removed from this static list, since the plugin machinery is now adding
them automatically.  autoconf fails if a file is duplicated in
AC_CONFIG_FILES().