I made this change so runner crosstests can be run in a Visual Studio command prompt, where dxcompiler.dll is in the path. `SONAME_LIBDXCOMPILER` is already defined even for crosstests, and there's no way to tell at compile time what its value should actually be for crosstest builds.
Right, I overlooked that the issue here is that we may want to use e.g. "libdxcompiler.so" for the regular build and "dxcompiler.dll" for the cross build. It turns out that in principle e.g. the Vulkan runner would have that issue as well, but conveniently the Vulkan runner can't work for cross builds.
In any case, something along the lines of Giovanni's proposal seems reasonable to me.