Make the build configuration under Bazel more correct.

[bcsgh]

• Expose JSON_USE_EXCEPTION and JSON_HAS_INT64 as Bazel config flags with defaults that match the existing Bazel build.
• Switch //:jsoncpp from using copts to defines for JSON_USE_EXCEPTION and JSON_HAS_INT64 so that rules that depend on it get the same config.
• Make src/test_lib_json/fuzz.cpp respect JSON_USE_EXCEPTION.
• #ifdef stuff that should only be used with JSON_USE_EXCEPTION.

Note:

See also #1601

Testing:

This builds with all 4 combinations:

for i in true false ; do
  for e in true false ; do
    bazel build --@//:use_exception=${e} --@//:has_int64=${i} ...
  done
done

[bcsgh]

I'm halfway tempted to try to make a version of this that fork()s and checks that the invocation dies. (e.g. a death test).

if (pid_t child = fork(); child) {
  int status = 0;
  waitpid(child, &status, 0);
  if (status == 0) {
    result_->addFailure(__FILE__, __LINE__, "Expected assertion: ", #expr);
  }
} else {
  expr;
  exit_(0);
}

Something like that might work, but I don't really have time to dig into all the ways that can be subtly wrong.