libtool.sh runs the command passed to it using "xcrun libtool".

It creates symbolic links for all input files with a content-hash appended
to their original name (foo.o becomes foo_{md5sum}.o). This is to circumvent
a bug in the original tool that arises when two input files have the same
base name (even if they are in different directories).

xcrun only runs on Darwin, so libtool.sh only runs on Darwin.

