Make sure asprintf() is declared if it's present.

Make sure asprintf() is declared if it's present.

GNU libc is a pain.  If you don't define _GNU_SOURCE, it doesn't declare
asprintf().  If you *do* define _GNU_SOURCE, the strerror_r() it
declares isn't POSIX-compliant.

We use asprintf() if present, so we need it to be declared; define
_GNU_SOURCE in ftmacros.h.

At configuration time, if we have strerror_r(), check whether it's
GNU-style or POSIX-style, and define different #defines for those two
cases.  Handle GNU-style strerror_r() and POSIX-style strerror_r()
differently in pcap_fmt_errmsg_for_errno().

Make sure everything that uses asprintf() includes ftmacros.h before
including stdio.h.