configure: Fix test with -Werror=implicit-function-declaration

The ipv6 auto-detect test in configure returns a false negative when
CFLAGS contains -Werror=implicit-function-declaration. (I have been
using this flag to detect code issues that would result in SEGVs on
x86_64-cygwin.)

Patch-by: Yaakov Selkowitz
Bug: http://curl.haxx.se/bug/view.cgi?id=1304