message: Fix recommending abnormal cmake usage in the build directions
  LLVM_DIR is an internally set variable by find_package. It's better to
  prioritize finding it through the standard search mechanisms, rather
  than diretly setting this.
diff: @@ -60,7 +60,7 @@ cmake -GNinja -DLLVM_ENABLE_RTTI=ON -DLLVM_ENABLE_EH=ON -DBUILD_SHARED_LIBS=ON -
  Alive2 should then be configured as follows:
  ```
  -cmake -GNinja -DLLVM_DIR=~/llvm/build/lib/cmake/llvm -DBUILD_TV=1 -DCMAKE_BUILD_TYPE=Release ..
  +cmake -GNinja -DCMAKE_PREFIX_PATH=~/llvm/build -DBUILD_TV=1 -DCMAKE_BUILD_TYPE=Release ..
  ```
  If you want to use Alive2 as a clang plugin, add `-DCLANG_PLUGIN=1` to the
  