please find attached an alternate submission which addresses open item

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.

Fabien Coelho