add dependency information for g++ linux builds

I noticed a couple of mentions where people got errors due to a rebuild fail (make clean && make all would fix it) and so have added dependency information to the makefile. The change I made should be fine for any g++ compiler, we may need to make the -MMD option optional if we support a compiler that doesn't handle this.

However there appears to be a similar dependency check already that doesn't actually link to anything for Linux / g++. Notably multiple references to create and require a .deps directory and to include from $(DEPDIR) which was never set. It is possible these are required for other platforms that set this in environmental flags or call make with certain parameters so I have left them alone until someone better informed comments.