[cmake] print warning if oms not enabled

CMakeLists.txt

@@ -289,6 +289,10 @@ if( ENABLE_LOCAL AND NOT MICROHTTPD_FOUND )
 Install microhttpd or call cmake -DMICROHTTPD_HOME=path_to_microhttpd_install")
 endif( ENABLE_LOCAL AND NOT MICROHTTPD_FOUND )
 
+if( ENABLE_OMS AND NOT OMS_SUPPORT)
+  message(WARNING "oms support not being enabled (libmbus not found)")
+endif( ENABLE_OMS AND NOT OMS_SUPPORT)
+
 # add clean-all target that removes the cached files from cmake as well (see e.g. issue #186)
 add_custom_target(clean-all
     COMMAND ${CMAKE_BUILD_TOOL} clean