RFC: wchar_t configuration simplifications

[Benjamin Kosnik <bkoz at redhat dot com>]

Yo people.

Here's an attempt to make the C99 configuration bits a little more sane.
It's not yet done, but the general shape seems clear. If anybody has any
great ideas, or if this looks better/decent, let me know. 

This part of configure is kind of a mess. Pull one thing, and everything
is changed. 

In general, the way things are going to work is to test all of C99
support, for all targets (native and cross). To do this, many of the
configure link tests for various "C" functions in <wchar.h> will be
converted to compile tests (see the XXX in acinclude.m4). That way, all
the C99 functionality will be tested in the same way. This will also
allow some simplifications of crossconfig.m4 that are not yet done.

However, one cool thing works with this arrangement is that libstdc++
can be configured to not use any C99 bits at all, and this builds/runs
the testsuite 100%.

I also need a solaris tester, both native and cross. Any volunteers?

-benjamin

testedx86/linux 
tested x86/linux --disable-c-mbchar 
tested x86/linux --disable-c99

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