Enable library targets for LDC compiler.

[dmakarov]

Library targets for LDC compiler. Without these targets DUB is useless with LDC.

[s-ludwig]

Thanks! Seems like nobody tried LDC with 0.9.21 so far... (btw. a possible workaround is to use dub --combined)

Just one question regarding the patch: did you test generating a shared library? From what I've read, -relocation-model=pic should also be necessary.

[dmakarov]

I believe ldc2 generates pic code when -shared is used (no need to specify relocation model explicitly), but no I haven't test shared lib target. Also ldc2 seems to have issues with the linker command when -shared is used. I'll look more into it, but for now perhaps it's good to enable these targets instead of asserting... The static lib generation definitely works.

Thanks!

[s-ludwig]

Yeah, shared libraries also still have other issues (e.g. when static dependencies are also involved), so it's low priority right now anyway. I'll merge it like this for now.