How can I build LDC v1.18.0?

[Sobaya]

I'm working on building LDC in tag v1.18.0.

I cloned LDC with `--recursive`  option, checked out tag v1.18.0, 
and execute cmake like below:

```
mkdir build
cd build
cmake ..
```

But I got the error in cmake execution:
```
CMake Error at /usr/lib/cmake/llvm/LLVMExports.cmake:37 (message):
   Some (but not all) targets in this export set were already 
defined.

   Targets Defined: FileCheck;not

   Targets not yet defined:
....
```

After investigating, I found the cause is that cmake target 
"FileCheck`" and "not" are already defined in 
"ldc/utils/CMakeLists.txt`".
I thought that "ldc/utils/CMakeLists.txt`" is written to be 
loaded after including LLVMConfig because there are some checks 
for existence of those targets, which is defined in LLVMConfig.
But actually "ldc/utils/CMakeLists.txt`" is loaded before 
LLVMConfig.
I cannot understand why this can be built.

What should I do for building LDC successfully?

My operating system is Arch Linux and LLVM version is 9.0.0.