Makefile changes

Following #59 here are two changes for the Makefile.

The change regarding PREFIX will create a dependency on GNU make or similar implementations, so YMMV.

I made the aliases since from my previous experiences the 'clean' target has always been connected to cleaning up the build directory after configuration/compilation, etc. and the uninstall gives a nice diametric incantation to the install target. (Since as a newcomer I do not know where the Makefile is used in the big picture apart from simple user installation I did not want to rename the targets but please do so instead if it makes sense...)