[PATCH] Round 2: Magic block for modules

Per feedback, here is an updated version. As was pointed out, the prior
version was checking stuff that either changed too often to be useful
or was never going to change at all. The error reporting is cleaned up
too, it now releases the module before throwing the error. It now only
checks four things:

Major version number (7.4 or 8.1 for example)
NAMEDATALEN
FUNC_MAX_ARGS
INDEX_MAX_KEYS

The three constants were chosen because:

1. We document them in the config page in the docs
2. We mark them as changable in pg_config_manual.h
3. Changing any of these will break some of the more popular modules:

FUNC_MAX_ARGS changes fmgr interface, every module uses this
NAMEDATALEN changes syscache interface, every PL as well as tsearch uses this
INDEX_MAX_KEYS breaks tsearch and anything using GiST.

I considered others but ultimatly rejected them. For example,
HAVE_INT64_TIMESTAMP, while changing the way timestamps are stored and
being configurable by a configure option, doesn't actually break
anything important (only the btree_gist example in contrib).

Any more comments?

Have a nice day,
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.