pgxs default installation + various fixes

Dear patchers,

This patch addresses open item "make pgxs install by default" in Bruce's
list.

(1) "make intall" installs everything.

(2) "make light-install" does not install pgxs and server dev stuff.
this is the previous version of "make install". This target maybe
of interest of packagers. If you want any other name, just tell
me what you want!

(3) the installation.sgml documentation reflects the new status...
but only with my poor English.

(4) I noticed that some pgxs related files were incidently installed
under "light-install", so I make sure that they are not.

(5) a minor stylistic fix in the pgxs makefile, so that it does not
have a double slash (/some/directory//sub/directory) in some paths.

What might be consider for future improvements,
but ISTM not mandatory for 8.0?

- maybe rename "install-all-headers" target, as it also install
scripts, makefiles and libraries useful for server developpement.
I would suggest "install-server-dev".

- make the contrib regressions work with pgxs. I'm not sure about what
is needed for that.

I'm available to fix any problem with this patch.

Have a nice day,

--
Fabien Coelho - coelho(at)cri(dot)ensmp(dot)fr