Making libstdc++-v3 configure faster

[Benjamin Kosnik]

Phil Edwards did some work on this. He's the dude in charge of the first 
round of configure changes, which did indeed speed things up.

> I think this might have been discussed, but configuring libstdc++-v3
> is very slow for me on Irix6.  It spends more time configuring than it
> does bootstrapping and running the testsuite combined.  It seems to

? I find this a bit hard to believe, but ok.

> spend a lot of time checking for the various math decl and linkage
> functions.  IIRC, someone mentioned that one reason it is slow is that
> is does a whole lot of AC_LANG_SAVE/AC_LANG_RESTORE calls.  Indeed,

You might want to search the mailing list archive for the commentary the 
first time this went around... it's either on the gcc or libstdc++ lists, 
and probably both.

-benjamin