Summary:
When using clang with libstdc++ `-latomic` must be explicitly specified
as part of the link command in order to get some of the symbols required
for C++ std::atomic support.

The old autoconf-based build did have this check (added in github commit
60a5636b, D2519083).  However this check was missing from the
CMake-based build.

Reviewed By: igorsugak

Differential Revision: D10405960

fbshipit-source-id: 37291edc5a387e2e966ece79241aa00312194cbd