Add workaround to building tests without all dependencies

The system tests can be build even if not all of the dependencies for
MaxScale are met.

server/modules/filter/dbfwfilter/CMakeLists.txt

@@ -24,6 +24,8 @@ if(BISON_FOUND AND FLEX_FOUND)
   if(BUILD_TESTS)
     add_subdirectory(test)
   endif()
+elseif (BUILD_SYSTEM_TESTS)
+  message(STATUS "Not building dbfwfilter when tests are built and both flex and bison were not found")
 else()
     message(FATAL_ERROR "Could not find Bison or Flex: ${BISON_EXECUTABLE} ${FLEX_EXECUTABLE}")
 endif()