Enable building on MacPorts based installation

In contrast to my previous pull request, this one has been tested from afresh on my MacPorts based installation. Please test on a Homebrew based installation as well.

Homebrew has the macros in /usr/local/share/aclocal;
MacPorts has the macros in /opt/local/share/aclocal;
I don't understand why the -I argument is needed with either of
these directories, since
aclocal --print-ac-dir returns this directory.
The AC_DIR is searched by default.
An -I argument directory is searched before AC_DIR
See:
http://www.slac.stanford.edu/comp/unix/package/rtems/doc/
html/automake/automake.info.Macro_search_path.html