CMake fails to detect weak symbol support on Mac OS X
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Description
During the CMake `configure` step, on Mac OS X 10.14, weak symbol support is not found:
```
-- Performing Test FOLLY_HAVE_WEAK_SYMBOLS
-- Performing Test FOLLY_HAVE_WEAK_SYMBOLS - Failed
```
The detection at https://github.com/facebook/folly/blob/master/CMake/FollyConfigChecks.cmake#L112 doesn't quite work for Mac OS X, using Clang 7.0.1 or AppleClang. Changing `weak` to `weak_import` as suggested by [Apple's docs](https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html#//apple_ref/doc/uid/20002378-106633) still fails.
Original CMake forum post when Folly got ported from Autotools to CMake to support this functionality: http://cmake.3232098.n2.nabble.com/detecting-attribute-weak-support-td7589507.html
CC: @calebmarchent
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