[Patch] Fix ac_c99_complex configury

[Benjamin Kosnik]

> 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.

Instead I propose that you just use the C++ compiler here, and fixup
the issue with backwards/complex.h.

Thoughts?

-benjamin