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(at)cri(dot)ensmp(dot)fr