[Patch] Fix ac_c99_complex configury

[Gabriel Dos Reis]

Paolo Carlini <pcarlini@suse.de> writes:

| Hi,
| 
| 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.

Should not we test both compilation and link?
The reason is that if we don't check for declaration, we might end up
with a conflicting declaration.

-- Gaby