Linker flags don't work.

script/ci_build.sh

@@ -4,12 +4,11 @@ set -ex
 
 if [[ "${CXX}" == clang* ]]; then
     export CXXFLAGS="-stdlib=libc++"
-    EXTRA="-DCMAKE_EXE_LINKER_FLAGS=-lc++abi -lc++"
 fi
 
 mkdir build && cd build
 
-cmake ${EXTRA} ..
+cmake ..
 make
 make unittest