Modify plugins.m4 configuration framework so that plugins which are
not built still get added to the source distribution during make dist.
This came up now because we can only build ibmdb2i on i5/OS, and we
can't bootstrap our source dist on that platform.  The solution is to
specify DIST_SUBDIRS containing all plugins, separate from SUBDIRS
which contains the plugins which are actually built.

This ibmdb2i code is from the ibmdb2i-ga3-src.zip file, with a patch
to plug.in to disable the plugin if the PASE environment isn't available.