configure: Check that "libtool" is not the MacOSX one

The "libtool" binary on MacOSX is not GNU libtool, and doesn't support
anything like the same set of command line options. Test whether we
have accidentally picked this up (by looking for whether it handles
the GNU --version switch), and discard it if so. The fallback machinery
for the "we don't have a libtool" case will work fine. This fixes a
failure in "make install" on MacOSX.

Reported-by: Peter Cheung <mcheung63@hotmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1367701071-6630-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
