Fix AC_PATH on mac os x with homebrew
by default, homebrew installs into /usr/local, while the autoconfigure script looks in /opt/local for autotools. This commit tests for the existence of an /opt/local directory, then falls back onto /usr/local if none is found.
Merge pull request #250 from timcowlishaw/fix_mac_ac_path

Fix AC_PATH on mac os x with homebrew