this message should be printed if option is not defined, not if it is

OFF

CMakeLists.txt

@@ -559,7 +559,7 @@ else (NOT CHAMELEON_SIMULATION)
         "   else set CHAMELEON_USE_CUDA=OFF. The same idea is applicable with MAGMA."
         "${ColourReset}")
     endif()
-    if(NOT CHAMELEON_USE_MAGMA)
+    if(NOT DEFINED CHAMELEON_USE_MAGMA)
         message(WARNING "${BoldBlue}"
         "In simulation mode CHAMELEON_USE_CUDA and CHAMELEON_USE_MAGMA should be set to"
         "   ensure that the user is aware of the version to be used. If Chameleon's"