removed handwritten targets in Makefile in favor of Makefile implicit rules

While looking into the gcc34 versus gcc48 issues at MPI CBG, I upgraded the Makefile. As you can see, there is an automated target (line 8) that builds targets complying to the simple rule "foo.c -> foo". Adding shared.c as a dependency was not required because it was included in any case inside the source code (make based timestamp checking is hence obsolete).