[Patch] Fix ac_c99_complex configury

[Mark Mitchell]

Benjamin Kosnik wrote:
>>the use of the new __builtin_clog exposed a serious weakness in our
>>configury: the check for ac_c99_complex (and then for
>>_GLIBCXX_USE_C99_COMPLEX), uses the C compiler together with
>>AC_TRY_COMPILE: we are basically checking for prototypes but C (vs C++)
>>doesn't care and this means that the test basically *always* passes. I
>>propose to simply use AC_TRY_LINK, here and I'm finishing regtesting the
>>below.
> 
> 
> You can't use any link tests in this macro, it's used by crosses and native toolchains.

I thought you just couldn't use run tests.  Why can't you use link tests?

-- 
Mark Mitchell
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