cilksan ease of use suggestions

[wheatman]

A minor thing I noticed when trying to create a minimal example for #47

If you use cilksan and forget to compile with -fopencilk you get a lot of linker errors instead of a clear message of the mistake.
I don't know if this could be easily detected, but if so it might help users.

error:

/usr/bin/ld: /home/ubuntu/OpenCilk-10.0.1-Linux/lib/clang/10.0.1/lib/x86_64-unknown-linux-gnu/libclang_rt.cilksan.a(driver.cpp.o): in function `__csan_init':
/home/neboat/opencilk/cilktools/cilksan/driver.cpp:214: undefined reference to `__cilkrts_is_initialized'
/usr/bin/ld: /home/ubuntu/OpenCilk-10.0.1-Linux/lib/clang/10.0.1/lib/x86_64-unknown-linux-gnu/libclang_rt.cilksan.a(driver.cpp.o): in function `init_internal':
/home/neboat/opencilk/cilktools/cilksan/driver.cpp:215: undefined reference to `__cilkrts_internal_set_nworkers'
/usr/bin/ld: /home/neboat/opencilk/cilktools/cilksan/driver.cpp:216: undefined reference to `__cilkrts_internal_set_force_reduce'
/usr/bin/ld: /home/ubuntu/OpenCilk-10.0.1-Linux/lib/clang/10.0.1/lib/x86_64-unknown-linux-gnu/libclang_rt.cilksan.a(locking.cpp.o): in function `__csan_pthread_mutex_lock':
/home/neboat/opencilk/cilktools/cilksan/locking.cpp:128: undefined reference to `__cilkrts_get_tls_worker'
/usr/bin/ld: /home/ubuntu/OpenCilk-10.0.1-Linux/lib/clang/10.0.1/lib/x86_64-unknown-linux-gnu/libclang_rt.cilksan.a(locking.cpp.o): in function `__csan_pthread_mutex_trylock':
/home/neboat/opencilk/cilktools/cilksan/locking.cpp:142: undefined reference to `__cilkrts_get_tls_worker'
/usr/bin/ld: /home/ubuntu/OpenCilk-10.0.1-Linux/lib/clang/10.0.1/lib/x86_64-unknown-linux-gnu/libclang_rt.cilksan.a(locking.cpp.o): in function `__csan_pthread_mutex_unlock':
/home/neboat/opencilk/cilktools/cilksan/locking.cpp:156: undefined reference to `__cilkrts_get_tls_worker'
/usr/bin/ld: /home/ubuntu/OpenCilk-10.0.1-Linux/lib/clang/10.0.1/lib/x86_64-unknown-linux-gnu/libclang_rt.cilksan.a(locking.cpp.o): in function `__csan_mtx_lock':
/home/neboat/opencilk/cilktools/cilksan/locking.cpp:169: undefined reference to `__cilkrts_get_tls_worker'
/usr/bin/ld: /home/ubuntu/OpenCilk-10.0.1-Linux/lib/clang/10.0.1/lib/x86_64-unknown-linux-gnu/libclang_rt.cilksan.a(locking.cpp.o): in function `__csan_mtx_trylock':
/home/neboat/opencilk/cilktools/cilksan/locking.cpp:183: undefined reference to `__cilkrts_get_tls_worker'
/usr/bin/ld: /home/ubuntu/OpenCilk-10.0.1-Linux/lib/clang/10.0.1/lib/x86_64-unknown-linux-gnu/libclang_rt.cilksan.a(locking.cpp.o):/home/neboat/opencilk/cilktools/cilksan/locking.cpp:200: more undefined references to `__cilkrts_get_tls_worker' follow
/usr/bin/ld: /tmp/del-7249a6.o: in function `main':
del.cpp:(.text+0x25e): undefined reference to `__csan_llvm_x86_avx2_gather_d_d'
clang-10: error: linker command failed with exit code 1 (use -v to see invocation)