Fixed nullopt static ocnstant not being compiled in.

It was used but we probably got away thanks to compiler optimizing it out.

common/CMakeLists.txt

@@ -138,7 +138,8 @@ set (COMMON_LIB_SRC_FILES
   ConfigurationFile.cpp
   SourcedParameter.cpp   
   Timer.cpp
-  UserIdentity.cpp)
+  UserIdentity.cpp
+  optional.cpp)
 
 add_library (ctacommon SHARED
   ${COMMON_LIB_SRC_FILES})

common/optional.cpp

@@ -23,4 +23,8 @@
 
 #include "optional.hpp"
 
-nullopt_t nullopt{};
+namespace cta{
+
+const cta::nullopt_t nullopt{};
+
+}
\ No newline at end of file

common/optional.hpp

@@ -28,7 +28,7 @@ struct nullopt_t {
     nullopt_t() {}
 };
 
-extern nullopt_t nullopt;
+extern const nullopt_t nullopt;
 
 template <class T> class optional
 {