add BUILD_WITH_CLANG flag

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

Ex.

```
export LLVM_DIR=/usr/src/clang-r487747
export LLVM_CONFIG=$LLVM_DIR/bin/llvm-config

BUILD_WITH_CLANG=true source build.sh
```

Author: joeyjiaojg

build.sh

@@ -12,6 +12,7 @@ jobs=$(echo $(nproc) || echo $(sysctl -n hw.physicalcpu))
 #########
 # VARs and Links
 ########
+CMAKE_EXTRA_FLAGS=""
 SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
 SVFHOME="${SCRIPT_DIR}"
 sysOS=$(uname -s)
@@ -222,10 +223,11 @@ BUILD_DIR="./${BUILD_TYPE}-build"
 
 rm -rf "${BUILD_DIR}"
 mkdir "${BUILD_DIR}"
+[ "${BUILD_WITH_CLANG}" != "" ] && CMAKE_EXTRA_FLAGS="-DCMAKE_CXX_COMPILER=$LLVM_DIR/bin/clang++ -DLLVM_DIR=$LLVM_DIR"
 cmake -D CMAKE_BUILD_TYPE:STRING="${BUILD_TYPE}" \
     -DSVF_ENABLE_ASSERTIONS:BOOL=true            \
     -DSVF_SANITIZE="${SVF_SANITIZER}"            \
-    -S "${SVFHOME}" -B "${BUILD_DIR}"
+    -S "${SVFHOME}" -B "${BUILD_DIR}" ${CMAKE_EXTRA_FLAGS}
 cmake --build "${BUILD_DIR}" -j ${jobs}
 
 ########