D: LTO with LDC
Nice that D is included.

For the compilation flags for LDC: turning on LTO is best done by passing the flag directly to LDC (i.e. not to the C compiler with `-Xcc`).
Better flags are: `-flto=full -defaultlib=phobos2-ldc-lto,druntime-ldc-lto`
With those flags, you get LTO also for the standard library and runtime library (although it is not much used in the program).