Include files for SPI are not installed

Certain include files are installed by src/include/Makefile and by
interfaces/libpq++/Makefile. However, they in turn include others that
are not installed, thus obviating the usefulness of the ones that are.

The missing files are these:

access/heapam.h
access/htup.h
access/relscan.h
access/rmgr.h
access/sdir.h
access/skey.h
access/strat.h
access/transam.h
access/tupdesc.h
access/tupmacs.h
access/xact.h
access/xlogdefs.h
access/xlog.h
access/xlogutils.h
catalog/pg_am.h
catalog/pg_attribute.h
catalog/pg_class.h
catalog/pg_language.h
catalog/pg_proc.h
catalog/pg_type.h
executor/execdefs.h
executor/execdesc.h
executor/executor.h
executor/hashjoin.h
executor/tuptable.h
nodes/execnodes.h
nodes/memnodes.h
nodes/nodes.h
nodes/params.h
nodes/parsenodes.h
nodes/pg_list.h
nodes/plannodes.h
nodes/primnodes.h
nodes/relation.h
pgconnection.h
pgdatabase.h
pgtransdb.h
rewrite/prs2lock.h
storage/block.h
storage/buffile.h
storage/buf.h
storage/bufmgr.h
storage/bufpage.h
storage/fd.h
storage/ipc.h
storage/item.h
storage/itemid.h
storage/itemptr.h
storage/lmgr.h
storage/lock.h
storage/off.h
storage/page.h
storage/relfilenode.h
storage/shmem.h
storage/spin.h
tcop/dest.h
tcop/pquery.h
tcop/tcopprot.h
tcop/utility.h
utils/builtins.h
utils/datetime.h
utils/datum.h
utils/fcache.h
utils/hsearch.h
utils/memutils.h
utils/nabstime.h
utils/numeric.h
utils/portal.h
utils/rel.h
utils/syscache.h
utils/timestamp.h
utils/tqual.h

The list can be regenerated with the attached script.

Example:
$ pg_includes /usr/local/pgsql/include