make sure we have either Linux or Apple or Win32

CMakeLists.txt

@@ -111,14 +111,19 @@ include(BornAgain/MakeLib) # fct MakeLib
 
 include(BornAgain/Directories)
 
-if(CMAKE_SYSTEM_NAME MATCHES Linux)
-    include(BornAgain/Linux)
-    include(BornAgain/NixInstall)
-elseif(APPLE)
-    include(BornAgain/MacOS)
+if(UNIX)
+    if(CMAKE_SYSTEM_NAME MATCHES Linux)
+        include(BornAgain/Linux)
+    elseif(APPLE)
+        include(BornAgain/MacOS)
+    else()
+        message(FATAL_ERROR "Operating system ${CMAKE_SYSTEM_NAME} not supported")
+    endif()
     include(BornAgain/NixInstall)
 elseif(WIN32)
     include(BornAgain/Windows)
+else()
+    message(FATAL_ERROR "Operating system ${CMAKE_SYSTEM_NAME} is neither UNIX nor WIN32")
 endif()
 
 include(BornAgain/Compiler)