A few Makefile changes to help with debian packaging

I ran into these problems when building with the debian package tools:

- Some of the debhelper tools were choking on the space character in the "stepmania 5.0" installation directory name, despite the use of quotes in the Makefile. Replacing the space with a dash got things working smoothly.

- There was no way for package tools to tell the Makefile to install to a package staging directory. Adding the conventional $(DESTDIR) variable fixed it, and doesn't interfere with manual builds.

- When running Make in parallel mode (-j), the install was failing because the target directory hadn't yet been created. Reordering two install commands fixed it. This has the additional benefit of allowing anyone to do parallel builds, which are much faster on multi-core machines.