Build macOS wheels with Xcode 6.4, supporting older macOS versions

The macOS wheels for Pillow 5.1.0 were built with Xcode 9.2, meaning 10.12 Sierra was the lowest supported version.

Prior to Pillow 5.1.0, Xcode 8 was used, supporting El Capitan 10.11.

Instead, Pillow 5.1.1 is built with the oldest available Xcode 6.4 to support at least 10.10 Yosemite.

Fix _i2f compilation with some GCC versions

For example, this allows compilation with GCC 4.8 on NetBSD.