Fix D test suite and add it to Travis CI

Octave and Javascript have started moving the examples to a common makefile and they are named example.mk - can you rename this one to example.mk.in. 

I don't think Octave nor this D makefile deals particularly well with having a common makefile for both C and C++ examples. I think the Octave approach is slightly better as the output is less messy, so unless you can think of an even better approach, can you this makefile to using make if statements instead of runtime shell if statements?