Fix build with Ninja generator

Summary:
We were hitting a bug in CMake's Ninja generator that was causing it to incorrectly attempt to escape the closing quote on `_HAS_AUTO_PTR_ETC=1`, so remove the quotes, as they aren't actually needed.
This also makes a few quality-of-life changes to make building Folly with Ninja slightly easier.

Closes: https://github.com/facebook/folly/issues/604
Reviewed By: yfeldblum

Differential Revision: D5134585

fbshipit-source-id: 3816e53774f7963b9b4fd8ddf68b83ff7afdefe1