Don't pick up libatomic from ports gcc if it happens to be installed.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -194,7 +194,7 @@ int main() {
     set(ATOMIC_LIBRARY atomic PARENT_SCOPE)
   endif()
 endfunction()
-check_atomic()
+# check_atomic()
 
 set(WINDOWS_WMAIN_COMPILE "")
 set(WINDOWS_WMAIN_LINK "")