/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/ is in libgcc-6-dev 6.3.0-18+deb9u1.
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..
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/cilk.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/cilk_api.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/cilk_api_linux.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/cilk_stub.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/cilk_undocumented.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/common.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/holder.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/hyperobject_base.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/metaprogramming.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_file.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_list.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_max.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_min.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_min_max.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_opadd.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_opand.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_opmul.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_opor.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_opxor.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_ostream.h
/usr/lib/gcc/x86_64-linux-gnu/6/include/cilk/reducer_string.h