Detect Windows perl linkage parameters in configure script.

This means we can now construct a configure test for the library
presence. Previously these parameters were only figured out at
build time in plperl's GnuMakefile.
Add explicit configure-time checks for perl.h and libperl.so.

Although most platforms seem to package Perl in such a way that these files
are present even in basic Perl installations, Debian does not.  Hence, make
an effort to fail during configure rather than build if --with-perl was
given and these files are lacking.  Per gripe from Josh Berkus.
Test linking libperl.so using only Perl's required libraries.

It appears that perl_embed_ldflags should already mention all the libraries
that are required by libperl.so itself.  So let's try the test link with
just those and not the other LIBS we've found up to now.  This should
more nearly reproduce what will happen when plperl is linked, and perhaps
will fix buildfarm member okapi's problem.
Last-gasp attempt to save libperl.so configure probe.

I notice that plperl's makefile adds the -I for $perl_archlibexp/CORE
at the end of CPPFLAGS not the beginning.  It seems somewhat unlikely
that the include search order has anything to do with why buildfarm
member okapi is failing, but I'm about out of other ideas.
Redesign the planner's handling of index-descent cost estimation.

Historically we've used a couple of very ad-hoc fudge factors to try to
get the right results when indexes of different sizes would satisfy a
query with the same number of index leaf tuples being visited.  In
commit 21a39de5809cd3050a37d2554323cc1d0cbeed9d I tweaked one of these
fudge factors, with results that proved disastrous for larger indexes.
Commit bf01e34b556ff37982ba2d882db424aa484c0d07 fudged it some more,
but still with not a lot of principle behind it.

What seems like a better way to address these issues is to explicitly model
index-descent costs, since that's what's really at stake when considering
diferent indexes with similar leaf-page-level costs.  We tried that once
long ago, and found that charging random_page_cost per page descended
through was way too much, because upper btree levels tend to stay in cache
in real-world workloads.  However, there's still CPU costs to think about,
and the previous fudge factors can be seen as a crude attempt to account
for those costs.  So this patch replaces those fudge factors with explicit
charges for the number of tuple comparisons needed to descend the index
tree, plus a small charge per page touched in the descent.  The cost
multipliers are chosen so that the resulting charges are in the vicinity of
the historical (pre-9.2) fudge factors for indexes of up to about a million
tuples, while not ballooning unreasonably beyond that, as the old fudge
factor did (even more so in 9.2).

To make this work accurately for btree indexes, add some code that allows
extraction of the known root-page height from a btree.  There's no
equivalent number readily available for other index types, but we can use
the log of the number of index pages as an approximate substitute.

This seems like too much of a behavioral change to risk back-patching,
but it should improve matters going forward.  In 9.2 I'll just revert
the fudge-factor change.
Extend and improve use of EXTRA_REGRESS_OPTS.

This is now used by ecpg tests, and not clobbered by pg_upgrade
tests. This change won't affect anything that doesn't set this
environment variable, but will enable the buildfarm to control
exactly what port regression test installs will be running on,
and thus to detect possible rogue postmasters more easily.

Backpatch to release 9.2 where EXTRA_REGRESS_OPTS was first used.