Use "long long int" when checking whether atomics are supported.

[vkalintiris]

Summary

Some targets implement 4-byte atomic related functions without calling
into libatomic, while for 8-byte atomics they need to make a function
call. By using a long long int we can identify such cases.

[jroelofs]

What happens on platforms that don't need the lib to make std::atomic<long long int> x; work?

[jyknight]

In D16614#336817, @jroelofs wrote:

What happens on platforms that don't need the lib to make std::atomic<long long int> x; work?

Then the first check succeeds, and they don't get -latomic added to the link line (HAVE_CXX_ATOMICS_WITHOUT_LIB gets set)

cmake/modules/CheckAtomic.cmake

 # atomic builtins are required for threading support.
 
 INCLUDE(CheckCXXSourceCompiles)
 
 # Sometimes linking against libatomic is required for atomic ops, if
 # the platform doesn't support lock-free atomics.
 
 function(check_working_cxx_atomics varname)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
   set(CMAKE_REQUIRED_FLAGS "-std=c++11")
   CHECK_CXX_SOURCE_COMPILES("
 #include <atomic>
-std::atomic<int> x;
+std::atomic<long long int> x;
 int main() {
   return x;
 }
 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
 endfunction(check_working_cxx_atomics)
 
 # This isn't necessary on MSVC, so avoid command-line switch annoyance