We never want LIBTOOL to mean a GNU libtool (which is a totally
different program) which might be present in $PATH before
/usr/bin/libtool, specifically /usr/local/bin, which by default (and
stupidly IMHO) is before /usr/bin on stock OS X.

Change-Id: I633a9fd275503f77fc1a0dd3d24a6776579792ba