Make targets for creating releases+release candidates

What this does
==============

* remove the check for .svn: No longer necessary since we moved to git.

* for the `release` target we simply reuse asf-dist-sign, and
additionally create a signed tag of the same name.

* for the `rel-candidate` target, we add a variable `RC` to Makefile.am
with default value of 0. This can be overriden from outside by calling

    make release-candidate RC=14

NOTE: tags are created with `-f`. If a tag of the same name existed on
your system, it will be overwritten. This is considered a feature.

What this does *not* do
=======================

We are not pushing any tags upstream, or uploading any tarballs
anywhere. This is in the responsibility of the Release Manager.

Likewise, we do *not* bump the version in configure.ac
reliably recreate the same tar-ball from the same content

if the content of the tree is the same, we now create the same
tarballs with the same and sha1sums. We do this, by fixing the mtime of
all objects in the tarball to that of our ./configure.ac

In order for the content to be the same, we no longer rely on simply
calling $(MAKE) asf-dist-sign with a different version, but rather have
special targets, which simply inject the $(RC) variable.

These targets are entirely convinience wrappers for Release managers.

NOTE the GPG signature will always different, because it's a cryptographic
signature. In order to release 4.2.16-rc7 as 4.2.16, we can either call

    make release

or, simply rename the tar-ball. The content of the tar-ball will be the
same, regardless.