Add libtool to OSX installs

on first build, I ran into this error:

    ...
    x test/test-walk-handles.c
    x test/test-watcher-cross-stop.c
    x uv.gyp
    x vcbuild.bat
    + glibtoolize
    autogen.sh: line 43: glibtoolize: command not found
    make: *** [.deps/usr/lib/libuv.a] Error 127

Fixed by installing "libtool" with brew.

I did not test the Port command but it looks like they have a "libtool" package too.