Fix AM_MAINTAINER_MODE as per guidelines

This shouldn't be the default mode, as universely agreed upon by
distributions.  It means that modifications to Makefile.am are not
automatically detected.  AM_MAINTAINER_MODE([enable]) is the recommended
solution, which disables it unless explicity requested during a
configure.
Merge pull request #3 from johnflux/configurefix

Fix AM_MAINTAINER_MODE as per guidelines