And infer CMake LDC_WITH_LLD automatically based on availability of LLVM 3.9+ with LLD headers & libs.
Results in a 7.5% bigger RelWithDebInfo ldc2.exe on Win64 with LLVM 3.9.1. LLD is currently enforced when building with LDC_WITH_LLD=ON. And LLD still doesn't support debuginfo (.pdb) generation for MSVC targets.