add option to build with clang

when set BUILD_WITH_CLANG=true, source build.sh will build with clang.

Ex.

```
export LLVM_DIR=/usr/src/clang-android
export LLVM_CONFIG=$LLVM_DIR/bin/llvm-config
CMAKE_CXX_COMPILER=${LLVM_DIR}/bin/clang++ source ./build.sh
```

Author: joeyjiaojg

build.sh

@@ -224,6 +224,8 @@ mkdir "${BUILD_DIR}"
 cmake -D CMAKE_BUILD_TYPE:STRING="${BUILD_TYPE}" \
     -DSVF_ENABLE_ASSERTIONS:BOOL=true            \
     -DSVF_SANITIZE="${SVF_SANITIZER}"            \
+    -DCMAKE_CXX_COMPILER:STRING="${CMAKE_CXX_COMPILER}" \
+    -DLLVM_DIR=${LLVM_DIR} \
     -S "${SVFHOME}" -B "${BUILD_DIR}"
 cmake --build "${BUILD_DIR}" -j ${jobs}