[v3] use stdc++.h.gch if possible

[Benjamin Kosnik]

Revised patch, does minimal check to see if pch can be built. I didn't
do a usage check of the generated .gch, since it's a precompiled math.h
include: not where problems have turned up in the past. Redoes the
include build rules conditionally, based on configure output. Blah blah
blah.

I'll let this ride for 12-24 hrs. Loren and Paolo seem to be
experimenting with PCH support as well. If this looks ok, and things
don't break, and people can't figure out a better way to do the autoconf
checks or make them more bullet-proof, I'd like to check this in as-is.

-benjamin 
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