on MacPorts need to enforce system /usr/bin/libtool for -static

error message: libtool: unrecognized option `-static
libtool: Try `libtool --help' for more information.
because on macports libtool is taken from /opt/local/bin.
-static is a xcode-only aberation.
Merge pull request #187 from rurban/dyncall-macports

on MacPorts need to enforce system /usr/bin/libtool for -static