OS X and ossp-uuid, next chapter

We already know that the uuid-ossp extension doesn't build OS X unless a
small patch is applied.

This has now gotten slightly worse after the Autoconf upgrade, because
it will now fail if a header is present but cannot be compiled.
(Previous versions would only warn. This is part of a decade-long
transition process Autoconf is doing.)

So now you get:

checking ossp/uuid.h usability... no
checking ossp/uuid.h presence... no
checking for ossp/uuid.h... no
checking uuid.h usability... no
checking uuid.h presence... yes
configure: WARNING: uuid.h: present but cannot be compiled
configure: WARNING: uuid.h: check for missing prerequisite headers?
configure: WARNING: uuid.h: see the Autoconf documentation
configure: WARNING: uuid.h: section "Present But Cannot Be Compiled"
configure: WARNING: uuid.h: proceeding with the compiler's result
configure: WARNING: ## ---------------------------------------- ##
configure: WARNING: ## Report this to pgsql-bugs(at)postgresql(dot)org ##
configure: WARNING: ## ---------------------------------------- ##
checking for uuid.h... no
configure: error: header file <ossp/uuid.h> or <uuid.h> is required for
OSSP-UUID

A possible patch is to hack up the uuid.h check to revert to the old
behavior; see attached.

I don't necessarily want to apply that, because it's an OS-specific hack
and it doesn't even work by itself unless we also patch the place where
the header is used (previously discussed). But I'll put it on record
here for future reporters and for the benefit of packagers.