add build utilities in default install

Dear patchers,

Please find attached my first submission for installing all necessary
build files (makefile, scripts...) by default.

 - for the old behavior, do a "make light-install"

 - "make install" installs headers and other necessary files
   such as scripts and makefiles, for postgresql extensions
   to be build.

 - these files are installed in "pg_config --insbuilddir"
   the actual directory can be modified during configure with
   "--with-insbuilddir=DIR" option. Default is LIBDIR/build

 - the installation documentation is updated.

 - I also added a "install-client-only" target so as to simplify the doc.


What is yet to be done:

 - check that all necessary files are really there...
   esp. wrt win32/cygwin

 - provide an example makefile (script?) to build extensions, and install it!
   I'm planning to do that later.

 - check that the documentation is clear enough.

It validates (make check does perform a "make install").
I'm obviously ready to update the patch if necessary (missing files,
other default directories...).

Have a nice day,

--
Fabien Coelho - coelho@cri.ensmp.fr