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