Fix issue #1238 make cmake find libatomic (#1406)

Summary:
this fixes cmake not finding libatomic at least on clearlinux 33470

Pull Request resolved: #1406

Reviewed By: Gownta

Differential Revision: D62074328

Pulled By: yfeldblum

fbshipit-source-id: 6c08e875c2a29f3755f52430fe823f585ff248e4

CMake/folly-deps.cmake

@@ -196,6 +196,7 @@ check_cxx_source_compiles("
 if(NOT FOLLY_CPP_ATOMIC_BUILTIN)
   list(APPEND CMAKE_REQUIRED_LIBRARIES atomic)
   list(APPEND FOLLY_LINK_LIBRARIES atomic)
+  set(ATOMIC_LIBRARY "atomic")
   check_cxx_source_compiles("
     #include <atomic>
     int main(int argc, char** argv) {