build: Switch fuzz builds to use Clang 5.0 sanitize=fuzzer

The old fuzzer location is deprecated; this also makes it almost trivial
to fuzz, provided that the clang is set up correctly... on Ubuntu 17.10,
a command sequence like this works now:

    sudo apt install clang-5.0
    sudo apt install libfuzzer-5.0
    sudo cp /usr/lib/llvm-5.0/lib/libFuzzer.a /usr/lib/libLLVMFuzzer.a
    CXX=clang++-5.0 make fuzz_parse