Skip rocksdb plugin if sources can not be fetched

Either we are building from a source package, in which case all sources
should be present, or we are building from a repository. The repository
needs to fetch the rocksdb submodule before building rocksdb.

storage/rocksdb/CMakeLists.txt

 # TODO: Copyrights
+
+MACRO(SKIP_ROCKSDB_PLUGIN msg)
+  MESSAGE_ONCE(SKIP_ROCKSDB_PLUGIN "Can't build rocksdb engine - ${msg}")
+  RETURN()
+ENDMACRO()
+
 IF(NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/rocksdb/Makefile AND GIT_EXECUTABLE)
   EXECUTE_PROCESS(COMMAND "${GIT_EXECUTABLE}" submodule init
                   WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}")
@@ -7,14 +13,9 @@ IF(NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/rocksdb/Makefile AND GIT_EXECUTABLE)
 ENDIF()
 
 IF (NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/rocksdb/Makefile")
-  MESSAGE(SEND_ERROR "Missing Makefile in rocksdb directory. Try \"git submodule update\".")
+  SKIP_ROCKSDB_PLUGIN("Missing Makefile in rocksdb directory. Try \"git submodule update\".")
 ENDIF()
 
-MACRO(SKIP_ROCKSDB_PLUGIN msg)
-  MESSAGE_ONCE(SKIP_ROCKSDB_PLUGIN "Can't build rocksdb engine - ${msg}")
-  RETURN()
-ENDMACRO()
-
 # We've had our builders hang during the build process. This prevents MariaRocks
 # to be built on 32 bit intel OS kernels.
 IF(CMAKE_SYSTEM_PROCESSOR MATCHES "i[36]86")