[racket-dev] package install mode

I have been thinking about how developers who build their own Racket
are more likely to want installation-wide packages instead of user- and
version-specific packages. This is particularly true for those of us
who work from the git respository.

Maybe the default for a build `configure'd without `--prefix' (or, more
generally, for a non-Unix-style build) should be installation-wide
package installs, while the default for our pre-built distributions
should be user- and version-specific installs.

The default mode would be determined by configuration information in
the installation-specific package area, and something like `raco pkg
config -i default-mode <mode>' could change an installation's default.

Does this sound like a good idea?