Introduce vcs-svn lib

Introduce vcs-svn lib

Teach the build system to build a separate library for the
upcoming subversion interop support.

The resulting vcs-svn/lib.a does not contain any code, nor is
it built during a normal build.  This is just scaffolding for
later changes.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>