Summary:
This change updates the CMake config to detect whether GCC's `-fcoroutines` flag is supported and enable it for the build. This is required to enable support for Folly coroutines.

Followed the instructions in GCC page about C++ coroutines to get them enabled.
https://gcc.gnu.org/wiki/cxx-coroutines

NOTE: This Wiki page used to indicate that exceptions had to be disabled to enable subroutines, but this only applied to experimental support for coroutines and it's no longer the case.

This was uncovered by the fbthrift build, which needs Folly coroutines to implement streaming. A new fbthrift test using a stream type uncovered this missing feature in our build on Fedora.

Reviewed By: yfeldblum

Differential Revision: D30026779

fbshipit-source-id: 3324c2fc52fe5db793bdaba9c81f41bfe6ef2242