* tools/dev/unix/build/Makefile.svn: Allow my custom build script, and by
   extension the OpenBSD buildbot, to build SVN's dependencies in parallel.
   This requires declaring build dependencies properly, and also clearing
   MAKEFLAGS when invoking 'make' in subjobs, because in parallel mode GNU
   make mode puts flags into MAKEFLAGS which BSD make doesn't like.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1545702 13f79535-47bb-0310-9956-ffa450edef68