[CMake] Treat suggested dune-modules as OPTIONAL.

RECOMMENDED created new categories in the summary.
Technically there are no differences.

cmake/modules/DuneMacros.cmake

@@ -94,10 +94,10 @@ macro(find_dune_package module)
   if(DUNE_FIND_REQUIRED)
     set(required REQUIRED)
     set_package_properties(${module} PROPERTIES TYPE REQUIRED)
-  else(DUNE_FIND_REQUIRED)
+  else()
     unset(required)
-    set_package_properties(${module} PROPERTIES TYPE RECOMMENDED)
-  endif(DUNE_FIND_REQUIRED)
+    set_package_properties(${module} PROPERTIES TYPE OPTIONAL)
+  endif()
   if(DUNE_FIND_VERSION MATCHES "(>=|=|<=).*")
     string(REGEX REPLACE "(>=|=|<=)(.*)" "\\1" DUNE_FIND_VERSION_OP ${DUNE_FIND_VERSION})
     string(REGEX REPLACE "(>=|=|<=)(.*)" "\\2" DUNE_FIND_VERSION_NUMBER ${DUNE_FIND_VERSION})