Implement option to use old C++ stdlib

We haven't fully switched all our projects to the new stdlib, and mixing the two implementations does not work.

This patch adds an option to build flatbuffers against the old C++03 stdlib.

Okay, I think I see what we're doing here now.

1. this only affects Apple platforms
2. this only affects `clang` (since the `-stdlib=libc++` flag is clang-specific)
3. we remove `-stdlib=libc++` as a default flag *unless* `USE_CXX03_STDLIB` is given

Still, I question not changing the `-std=c++11` flag here, if the entire reason of this PR is to support c++03. So, either I'm still confused about how your change is accomplishing the task, or the change is not complete?
--ap