Change Makefile to invoke g++ -MM for dependencies

Change Makefile to invoke g++ -MM for dependencies

The g++ -MM $(SOURCES) creates dependency list (without system deps) for
all the sources in a format suitable for Makefile.

I don't like using g++ just after making use of clang++ explicit as I
don't know whether both of those are usually present on PCs.
The solution would probably be to provide the g++ incompatible flag
conditionally when c++ == clang++.