* Update Autotools.gitignore

Add ignore-rule for Makefiles generated by configure
(directly by config.status)

* Add some descriptive comment.

Add more descriptive comment to explain why Makefile should be ignore.

These files are created when running `autoreconf`; however they don't need to be added to version control.

add files generated by the autotool m4.

See the full list of scripts documented in AC_CONFIG_AUX_DIR in https://www.gnu.org/software/automake/manual/html_node/Optional.html

The file stamp-h1 only contains a timestamp for config.h - actually the
files timestamp itself serves with this information. Therefore it
actually cannot be of interest not to ignore it.