[cmake] Allow code generation system tests to depend on additional files

I have the use case of a boundary condition file, whose changes should retrigger
the code generation process.

cmake/modules/GeneratedSystemtests.cmake

@@ -5,7 +5,7 @@ function(dune_add_formcompiler_system_test)
   # parse arguments
   set(OPTION DEBUG NO_TESTS ANALYZE_GRID)
   set(SINGLE INIFILE BASENAME SCRIPT UFLFILE SOURCE)
-  set(MULTI CREATED_TARGETS)
+  set(MULTI CREATED_TARGETS DEPENDS)
   cmake_parse_arguments(SYSTEMTEST "${OPTION}" "${SINGLE}" "${MULTI}" ${ARGN})
 
   if(SYSTEMTEST_UNPARSED_ARGUMENTS)
@@ -58,7 +58,7 @@ function(dune_add_formcompiler_system_test)
     add_generated_executable(TARGET ${tname}
                              UFLFILE ${SYSTEMTEST_UFLFILE}
                              INIFILE "${CMAKE_CURRENT_BINARY_DIR}/${inifile}"
-                             DEPENDS ${SYSTEMTEST_INIFILE}
+                             DEPENDS ${SYSTEMTEST_INIFILE} ${SYSTEMTEST_DEPENDS}
                              EXCLUDE_FROM_ALL
                              ${SOURCE}
                              ${ANALYZE_GRID_STR}