[cmake] missing include directive.

`CMake:3.21.0` complains about undefined `check_cxx_source_compiles` command
```
CMake Error at CMakeLists.txt:154 (check_cxx_source_compiles):
Unknown CMake command "check_cxx_source_compiles".
Call Stack (most recent call first):
CMakeLists.txt:508 (config_with_llvm)
```

Looks like missing `INCLUDE(CheckCXXSourceCompiles)` Quick fix:
```diff
diff --git a/CMakeLists.txt b/CMakeLists.txt
index fab5907..978c940 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.tx
@@ -156,6 +151,7 @@ function (config_with_llvm)

set (CMAKE_REQUIRED_INCLUDES ${LLVM_INCLUDE_DIRS})
set (CMAKE_REQUIRED_LIBRARIES LLVMObject)
+ INCLUDE(CheckCXXSourceCompiles)
check_cxx_source_compiles(
"
#include \"llvm/Object/ELFObjectFile.h\"
```