Fix 'make crystal', .PHONY targets, don't assume . is in PATH.

Invoking "./crystal" doesn't seem to work anymore so I removed that from the makefile and simplified it.

Also the ".PHONY" target was missing, this is necessary for make targets that don't correspond to generated output files. Lastly it doesn't assume "." is in the PATH anymore (some consider "." in the PATH as a security risk).