configure / util: Auto-detect the availability of openpty()

configure / util: Auto-detect the availability of openpty()

Recent versions of Solaris (v11.4) now feature an openpty() function,
too, causing a build failure since we ship our own implementation of
openpty() for Solaris in util/qemu-openpty.c so far. Since there are
now both variants available in the wild, with and without this function
(and illumos is said to not have this function yet), let's introduce a
proper HAVE_OPENPTY define for this to fix the build failure.

Message-Id: <20200702143955.678-1-thuth@redhat.com>
Tested-by: Michele Denber <denber@mindspring.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>