Switched the $(CXX) of test/Makefile to clang BUILD IS STILL FAILING DUE TO LINKER ERROR
Make clean does now run the clean target on all Makefiles in the subdirectories again

Just to be sure everything is properly cleaned
Fixed issue in the linking process (missing libm)

The omnet++ part and the tests did not compile well because it seems
like clang does not add the libm (math library) to the linker flags by default
Merge branch 'clangInTests' into develop