On Darwin, only use /usr/bin/libtool if libtool is not Apple.
On Darwin, only use /usr/bin/libtool if libtool is not Apple.

The original change was to always use /usr/bin/libtool on Darwin,
in order to avoid using a GNU libtool installed by the user in the
path ahead of Apple's libtool.  However someone might install a
more recent Apple libtool ahead of /usr/bin/libtool.  This commit
checks to see if libtool is Apple, and uses /usr/bin/libtool if it
isn't.

Streamlet/ZLibWrap,Streamlet/ZLibWrap,Streamlet/ZLibWrap,Streamlet/ZLibWrap,Streamlet/ZLibWrap