Makefile.am: fixed other custom rules to avoid writing into unexistent directory.

Makefile.am: fixed other custom rules to avoid writing into unexistent directory.

The fixed rules do not presently trigger any build errors: target directory
is created by configure as it need to put autogenerated files doc/manpage.rst
and doc/help.rst in it.

However, this behaviour is occasional: if one removes the .in files, build
failure would be unmasked. So it makes sense to ensure that target directory
exists.