This allows to statically set which version of FreeBSD to compile for
using DFLAGS, choices are FreeBSD 10, 11 or 12. If unset, the default is
to use the same version as the host, or fallback to version 11 if
compiling on a different host, or using an old D compiler to build DMD.
To be decided is whether there should be a dynamic way of detecting the
FreeBSD OS version as well, such as by using getosreldate().
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.
