Rewrite installation makefile rules without for loops

Rewrite installation makefile rules without for loops

install-sh can install multiple files at once, so for loops are not
necessary.  This was already changed for the rest of the code some
time ago, but pgxs.mk was apparently forgotten, and the obsolete
coding style has now been copied to the PLs as well.

This also fixes the problem that the for loops in question did not
catch errors.