m4: create new directory for clean autogen.sh run

Create a m4/.gitignore file saying that all *.m4 files in the
directory should be ignored.
start a gitignore

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Radu Voicilas <rvoicilas@gmail.com>
Remove m4/ from .gitignore