Autoconf: Fix formatting of IPv6 results. [skip appveyor]

Output of the IPv6 part of ./configure used to look OK on Linux:

checking whether the operating system supports IPv6... yes
checking ipv6 stack type... linux-glibc

But it was not OK on most other systems, for example, on FreeBSD:

checking whether the operating system supports IPv6... yes
checking ipv6 stack type... checking how to run the C preprocessor... cc
  -E
checking for grep that handles long lines and -e... /usr/bin/grep
checking for egrep... /usr/bin/grep -E
kame
You do not have inet6 library, using libc

Get the sequence of these messages right.  Now on Linux this is:

checking whether the operating system supports IPv6... yes
checking how to run the C preprocessor... gcc -E
checking for egrep... (cached) /usr/bin/grep -E
checking ipv6 stack type... linux-glibc

And on FreeBSD it is:

checking whether the operating system supports IPv6... yes
checking how to run the C preprocessor... cc -E
checking for grep that handles long lines and -e... /usr/bin/grep
checking for egrep... /usr/bin/grep -E
checking ipv6 stack type... kame
You do not have inet6 library, using libc