[v3] extc++.h guards for iconv

[Benjamin Kosnik]

As discussed. This should allow bootstrap and testing of libstdc++ on
systems without iconv.

It looks like other problem areas on this patch are:
1) --with-iconv-prefix and negations
2) testsuite issues with darwin/solaris

These issues are not fixed in this patch.

tested x86/linux

-benjamin
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