Proposal: Stop running './gen-make.py -t dsp' for release tarballs+zipfiles

Currently, when in release mode, autogen.sh generates windows build files.

I think we should stop doing this, as the generated files hardcode the
release manager's own absolute (Unix!) paths, and additionally, have
variations caused by the release manager's availability of certain
dependencies. For example, rc4 contains
build/win32/msvc-dsp/libsvn_fs_base_msvc.dsp, but on my machine, the
windows build isn't outputting that file.

We *already* tell Windows people to run gen-make.py as part of the
build. Therefore, I presume the practice of running it from autogen.sh
in release mode is either obsolete, or entirely misguided.

Max.