build system: add a .gitignore

Ignore all files generated by running autoconf, ./configure, and make,
and the debian build, and editor backup files.

(The files generated by running autoheader are committed and therefore
we don't add them to .gitignore.  There is probably a mistake here,
which will be fixed later in this series.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Signed-off-by: Richard Kettlewell <richard@greenend.org.uk>