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.

The above if-guard seems to be backwards, no? That is, this appears to add `-stdlib=libc++` to the flags only if `USE_CXX03_STDLIB` is *not* defined.

Also, what is the behaviour of CXX when using both `-std=c++11` and `-stdlib=libc++`?
--ap