OSX build documentation updates, cleanups, warning suppression.

 - build-osx.md has some additions pertaining to Homebrew dependencies required to build the project ('autoconf' and 'automake')
 - build-osx.md suggests prepending BDB_INCLUDE_PATH to the system's PATH as the current system may have an existing version (other than the one installed by Homebrew) and this can lead to build errors.
 - silenced a few unused variable warnings seen during compilation.