This is slower than the original coding but avoids the problem of
including files in an unpredictable order.  Aside from being more
trustworthy, we can get rid of some exclusions that were formerly
made for what turn out to be ordering or re-inclusion problems.

I also modified it to include libpq's exported files in the check.
ecpg should be included as well, but I'm unclear on which ecpg .h
files are meant to be included by clients.

gram.h has ordering dependencies, which are satisfied when it's included
from gramparse.h, but might not be if it's pulled in directly.

This alters various incidental uses of C++ key words to use other similar
identifiers, so that a C++ compiler won't choke outright.  You still
(probably) need extern "C" { }; around the inclusion of backend headers.

based on a patch by Kurt Harriman <harriman@acm.org>

Also add a script cpluspluscheck to check for C++ compatibility in the
future.  As of right now, this passes without error for me.