[Patch] Fix libstdc++/8610 (draft2)

[Paolo Carlini]

Hi,

this is the second iteration. Basically:

  1- Incorporates Benjamin's suggestions.
  2- Fixed a serious bug in the autoconf test for LFS:
     actually we need AC_TRY_LINK, not just COMPILE!

I took the occasion to robustify two other pre-existing tests,
changing those too to AC_TRY_LINK (consistently with S_ISREG and
S_IFREG).

I have regtested the patch completely both LFS and not on x86/linux,
also carried out some basic LFS tests such as test64.cc, that I
attached yesterday.

I'm planning to regtest and do basic tests on x86_64 too, then, if
I don't hear more comments in say, 24 hours, will commit.

Paolo.

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