HOST_DC is more self-descriptive I think; DMD & D_VERSION are rather
misleading for D host compiler and its version.
Also get rid of the `install_d` arg and use HOST_DC directly.
Based on infrastructure in ci.sh (also used by Semaphore CI), but
breaking up the testsuite into separate steps; I find that much better.
Also testing all 3 host compilers - DMD, LDC and GDC.
Unlike Semaphore, it doesn't rebuild itself and skips the dub package
test (not sure what that is).
A 32-bit FreeBSD job might be a worthwhile addition for the future.
