Use Common.Makefile for the C makefile. Fixes #19
Fixing the name clash problems in the c backend (fixes #20)
Fixes a small bug in C makefiles
Add tests for the C backend

Also refactor the system tests: instead of testing all backends at once
in one test-suite, give each backend its own test suite.
The benefit of that is that we can use flags in cabal to deactivate test
suite if need be