Bugfix and new feature for PGXS

I've rebased the current set of pending patches I had, to fix pgxs and added 2
new patches.

Bugfixes have already been presented and sent in another thread, except the
last one which only fix comment in pgxs.mk.

The new feature consists in a new variable to allow the installation of
contrib header files.
A new variable MODULEHEADER can be used in extension Makefile to list the
header files to install.

The installation path default to $includedir/$includedir_contrib/$extension.
2 new variables are set to define directories: $includedir_contrib and
$includedir_extension, the former default to include/contrib and the later to
$includedir_contrib/$extension ($extension is the name of the extension).

This allows for example to install hstore header and be able to include them
in another extension like that:

# include "contrib/hstore/hstore.h"

For packagers of PostgreSQL, this allows to have a postgresql-X.Y-contrib-dev
package.
For developers of extension it's killing the copy-and-paste-this-function
dance previously required.

I've not updated contribs' Makfefile: I'm not sure what we want to expose.

I've 2 other patches to write to automatize a bit more the detection of things
to do when building with USE_PGXS, based on the layout. Better get a good
consensus on this before writing them.

Bugfix:
0001-fix-SHLIB_PREREQS-when-building-with-USE_PGXS.patch
0002-Create-data-directory-if-extension-when-required.patch
0003-set-VPATH-for-out-of-tree-extension-builds.patch
0004-adds-support-for-VPATH-with-USE_PGXS.patch
0006-Fix-suggested-layout-for-extension.patch

New feature:
0005-Allows-extensions-to-install-header-file.patch

I'll do a documentation patch based on what is accepted in HEAD and/or
previous branches.
--
Cédric Villemain +33 (0)6 20 30 22 52
http://2ndQuadrant.fr/
PostgreSQL: Support 24x7 - Développement, Expertise et Formation