more massaging on pgxs postresql extension infrastructure

Dear patchers,

after Peter's massaging on pgxs, I think the infrastructure deserves
some more massaging because:

(a) some files are missing (namely libpgport.? needed by pgbench for
instance, and I guess possibly by others).

(b) I think it is a key feature that one should be able to compile
contrib with the already installed postgresql, without having
to reconfigure.

so I submit this new patch. I've tried to preserve Peter modifications
to my initial submissions, plus:

(1) add libpgport installation under "install-all-headers" target

(2) all contrib Makefiles can ALSO use of the dynamic pgxs stuff
=> no direct inclusion of Makefile.global (which is generated by configure)
=> USE_PGXS macro enables that ("make USE_PGXS=1 install"), otherwise
it is just as before the patch, the static "configured"
infrastructured is used.

If there are still issues, which is perfectly possible, I'm really willing
to fix them while preserving the ability to compile postgresql contribs
with an already installed postgresql.

Basically it works for me.

Thanks in advance,

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