Check if external library is needed to have atomic support.

Akonadi is using atomic feature. Not every platform has direct support
for it. For some libatomic is needed. CheckAtomic from
extra-cmake-modules make sure, test this feature and add the library if
needed.

Fixes: https://bugs.debian.org/961647
(cherry picked from commit 24b239e1)

CMakeLists.txt

@@ -24,6 +24,7 @@ include(ECMGeneratePriFile)
 
 include(ECMSetupVersion)
 include(FeatureSummary)
+include(CheckAtomic)
 include(CheckIncludeFiles)
 include(ECMQtDeclareLoggingCategory)
 include(CheckSymbolExists)