Keep protobuf2 as the default for now

Author: dstoup

CMake/fletch-tarballs.cmake

@@ -494,7 +494,7 @@ endif()
 if (WIN32)
   set(Protobuf_SELECT_VERSION "3.9.0" CACHE STRING "Select the  version of ProtoBuf to build.")
 else()
-  set(Protobuf_SELECT_VERSION "3.9.0" CACHE STRING "Select the  version of ProtoBuf to build.")
+  set(Protobuf_SELECT_VERSION "2.5.0" CACHE STRING "Select the  version of ProtoBuf to build.")
   set_property(CACHE Protobuf_SELECT_VERSION PROPERTY STRINGS "2.5.0" "3.9.0")
 endif()