[plt-scheme] PLT Scheme v204 available

I had problems to run "make install".  Since there is a file called
"install" in the top-level directory the make command thinks that the
target "install" is up to date.  I think that either the script
"install" should be renamed to "install.sh" or the following target
should be added to the Makefile:

.PHONY : install

This works with GNU make.  I do not know if other make implementations
support .PHONY.