I'm seeing the current fuzzer build fail (during `cmake`) like so:

```
-- Performing Test IS_FSANITIZE_FUZZER_NO_LINK_SUPPORTED
-- Performing Test IS_FSANITIZE_FUZZER_NO_LINK_SUPPORTED - Failed
CMake Error at cmake/AddCFlagIfSupported.cmake:17 (message):
  Required flag -fsanitize=fuzzer-no-link is not supported
Call Stack (most recent call first):
  fuzzers/CMakeLists.txt:6 (add_c_flag)
```

The cmake log output contains something like so:

```
        /src/aflplusplus/libAFLDriver.a(aflpp_driver.o): in function `main':
        aflpp_driver.c:(.text+0x11b): undefined reference to `LLVMFuzzerTestOneInput'
        clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

I haven't figured out exactly what's happening, but I believe that
once line 5 has added `-fsanitize=fuzzer` to `CFLAGS`, future compile-
tests **also** use it during linking. This in turn pulls in the fuzzer
`main`, which expects an `LLVMFuzzerTestOneInput` symbol, and thus
fails.

Instead, just add `-fsanitize=fuzzer-no-link` to CFLAGS (as suggested
[by the documentation][libfuzzer]), and then use `-fsanitize=fuzzer`
only for linking the fuzzer targets. At least in my environment, this
results in a working fuzzer build.

[libfuzzer]: https://llvm.org/docs/LibFuzzer.html#fuzzer-usage