ENV: default CFLAGS to bottle values.

[MikeMcQuaid]

This reduces the deviation in compilation flags on a given OS between users and between bottling/not bottling to provide a consistent behaviour for debugging and support.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

---