modules: don't unset MAGNUM_*APPLICATION_* vars if they stay the same.

Use case: user is calling find_package(Magnum REQUIRED Sdl2Application) in
a toplevel dir and in a subdir. The second call would unset the
MAGNUM_APPLICATION_LIBRARIES which is not correct, because it still is just
that one application.

Also, now instead of unsetting the variables are set to -NOTFOUND so they
produce an error when used as libraries/include directories for a target.

modules/FindMagnum.cmake

@@ -561,17 +561,17 @@ foreach(component ${Magnum_FIND_COMPONENTS})
             if(NOT DEFINED MAGNUM_WINDOWLESSAPPLICATION_LIBRARIES AND NOT DEFINED MAGNUM_WINDOWLESSAPPLICATION_INCLUDE_DIRS)
                 set(MAGNUM_WINDOWLESSAPPLICATION_LIBRARIES ${MAGNUM_${_COMPONENT}_LIBRARIES})
                 set(MAGNUM_WINDOWLESSAPPLICATION_INCLUDE_DIRS ${MAGNUM_${_COMPONENT}_INCLUDE_DIRS})
-            else()
+            elseif(NOT MAGNUM_WINDOWLESSAPPLICATION_LIBRARIES STREQUAL MAGNUM_${_COMPONENT}_LIBRARIES AND NOT MAGNUM_WINDOWLESSAPPLICATION_INCLUDE_DIRS STREQUAL MAGNUM_${_COMPONENT}_INCLUDE_DIRS)
-                unset(MAGNUM_WINDOWLESSAPPLICATION_LIBRARIES)
+                set(MAGNUM_WINDOWLESSAPPLICATION_LIBRARIES MAGNUM_WINDOWLESSAPPLICATION_LIBRARIES-NOTFOUND)
-                unset(MAGNUM_WINDOWLESSAPPLICATION_INCLUDE_DIRS)
+                set(MAGNUM_WINDOWLESSAPPLICATION_INCLUDE_DIRS MAGNUM_WINDOWLESSAPPLICATION_INCLUDE_DIRS-NOTFOUND)
             endif()
         elseif(${component} MATCHES ".+Application")
             if(NOT DEFINED MAGNUM_APPLICATION_LIBRARIES AND NOT DEFINED MAGNUM_APPLICATION_INCLUDE_DIRS)
                 set(MAGNUM_APPLICATION_LIBRARIES ${MAGNUM_${_COMPONENT}_LIBRARIES})
                 set(MAGNUM_APPLICATION_INCLUDE_DIRS ${MAGNUM_${_COMPONENT}_INCLUDE_DIRS})
-            else()
+            else(NOT MAGNUM_APPLICATION_LIBRARIES STREQUAL MAGNUM_${_COMPONENT}_LIBRARIES AND NOT MAGNUM_APPLICATION_INCLUDE_DIRS STREQUAL MAGNUM_${_COMPONENT}_INCLUDE_DIRS)
-                unset(MAGNUM_APPLICATION_LIBRARIES)
+                set(MAGNUM_APPLICATION_LIBRARIES MAGNUM_APPLICATION_LIBRARIES-NOTFOUND)
-                unset(MAGNUM_APPLICATION_INCLUDE_DIRS)
+                set(MAGNUM_APPLICATION_INCLUDE_DIRS MAGNUM_APPLICATION_INCLUDE_DIRS-NOTFOUND)
             endif()
         endif()