[!306] [cmake][bugfix] Fix handling of absolute paths for UFL files

Merge branch 'bugfix/absolute-path-to-uflfile' into 'master'

ref:extensions/dune-codegen This fixes [#143]

See merge request [extensions/dune-codegen!306]

  [#143]: gitlab.dune-project.org/NoneNone/issues/143
  [extensions/dune-codegen!306]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/306


Closes #143

cmake/modules/DuneCodegenMacros.cmake

@@ -116,7 +116,7 @@ function(add_generated_executable)
   if(NOT GEN_UFLFILE)
     message(FATAL_ERROR "Need to specify the UFLFILE parameter for add_generated_executable")
   endif()
-  if(NOT IS_ABSOLUTE GEN_UFLFILE)
+  if(NOT IS_ABSOLUTE ${GEN_UFLFILE})
     set(GEN_UFLFILE ${CMAKE_CURRENT_SOURCE_DIR}/${GEN_UFLFILE})
   endif()
   if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${GEN_INIFILE})