[v3] use AM_ICONV

[Benjamin Kosnik]

This removes all non-standard iconv configury from libstdc++, and 
instead uses the AM_ICONV bits from top level. Like gcc, like java, etc.

It is hoped that this is the correct long-term solution to the iconv 
configury problems that the recent patches for libstdc++/28125 revealed. 
However, I will just try it out on mainline for now, and if that works 
then I will only move it over to gcc-4_2-branch, as it seems quite big.

While doing this, I uncovered a mess in the testsuite....

Two years ago, when I moved out the codecvt specializations from std:: 
to __gnu_cxx, I did an awful job. There's no way around it. Here are the 
following fixes: correct the docs, remove the cruft from the config and 
testsuite, make sure the test are run. I believe this is not a 
"complete" transition.

In particular, we need __gnu_cxx::enc_filebuf, if the (existing) tests 
in the testsuite are going to test for it....

tested x86/linux


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