make check in contrib

I noticed again that make check in contrib doesn't work, so here is a
patch to fix it. Perhaps someone wants to fill in the Windows support
for it. Naturally, this works only for contrib itself, not for external
packages that use pgxs.

A secondary issue that actually led to this: I was preparing a Debian
package for some module^Wextension^W^Hthing that uses pgxs. The Debian
packaging tools ("dh", to be exact, for the insiders) have this
convenience that by default they examine your makefile and execute the
standard targets, if found, in order. So it runs make all, make check,
which then fails because of

check:
@echo "'make check' is not supported."
@echo "Do 'make install', then 'make installcheck' instead."
@exit 1

You can override this, but it still means that everyone who packages an
extension will have to re-figure this out.

So while this message might be moderately useful (although I'm not sure
whether it's guaranteed that the suggestion will always work), I'd
rather get rid of it and not have a check target in the pgxs case.

Comments?