pgxs default installation + various fixes - v2

Dear patchers,

please find attached an alternate submission which addresses open item
"make pgxs install by default". It is up to the committers to chose.

(1) there is only one "install" target. no more "install-all-headers".
it simplifies/changes several makefiles.

(2) the documentation reflects the change.

(3) a minor fix on pgxs to use a nicer patch without a double slash.

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

A possible improvement wrt pgxs, maybe not mandatory for 8.0, would be to
allow regression tests, but again I'm not sure about what is needed and
what are the implications. I have to investigate, and I don't have much
time right now...

Have a nice day,

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