configure: define macro for fuzzing builds

Sometimes we want to disable certain functionality or otherwise modify
code for fuzzing. The FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION macro is
a standard way to accomplish this.

See https://llvm.org/docs/LibFuzzer.html#fuzzer-friendly-build-mode.

configure

@@ -174,7 +174,7 @@ set_defaults()
 	fi
     fi
     if [ "$FUZZING" != 0 ]; then
-	FUZZFLAGS="-fsanitize=fuzzer-no-link"
+	FUZZFLAGS="-fsanitize=fuzzer-no-link -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION"
 	CSANFLAGS="$CSANFLAGS $FUZZFLAGS"
     fi
     echo CSANFLAGS = $CSANFLAGS